diff --git a/.github/workflows/busted.yml b/.github/workflows/busted.yml
index bc57a55..efef0bd 100644
--- a/.github/workflows/busted.yml
+++ b/.github/workflows/busted.yml
@@ -53,7 +53,7 @@ jobs:
           LD_PRELOAD: ${{ matrix.os == 'ubuntu-latest' && '/usr/lib/x86_64-linux-gnu/libasan.so.6:/usr/lib/x86_64-linux-gnu/libstdc++.so.6:/usr/lib/x86_64-linux-gnu/libubsan.so.1' || '' }}
 
       - name: Run gcov
-        if: success() && ${{ startsWith(matrix.os, 'ubuntu-') }}
+        if: success() && startsWith(matrix.os, 'ubuntu-')
         run: |
           gcov -p -b -s"$(pwd)" -r *.gcda
 
diff --git a/ckiwi/ckiwi.cpp b/ckiwi/ckiwi.cpp
index be6c013..410656a 100644
--- a/ckiwi/ckiwi.cpp
+++ b/ckiwi/ckiwi.cpp
@@ -326,7 +326,7 @@ bool kiwi_solver_has_constraint(const KiwiSolver* s, KiwiConstraint* constraint)
 }
 
 const KiwiErr* kiwi_solver_add_edit_var(KiwiSolver* s, KiwiVar* var, double strength) {
-   return wrap_err(s, var, [strength](auto& s, auto&& v) {
+   return wrap_err(s, var, [strength](auto&& s, auto&& v) {
       s.addEditVariable(Variable(v), strength);
    });
 }
diff --git a/luakiwi/luakiwi.cpp b/luakiwi/luakiwi.cpp
index bf63335..89ffa8a 100644
--- a/luakiwi/luakiwi.cpp
+++ b/luakiwi/luakiwi.cpp
@@ -248,6 +248,7 @@ KiwiTerm* term_new(lua_State* L) {
 
 inline KiwiExpression* expr_new(lua_State* L, int nterms) {
    auto* expr = static_cast<KiwiExpression*>(lua_newuserdata(L, KiwiExpression::sz(nterms)));
+   expr->term_count = 0;
    expr->owner = nullptr;
    push_type(L, EXPR);
    lua_setmetatable(L, -2);
@@ -929,9 +930,8 @@ int lkiwi_expr_new(lua_State* L) {
 
    auto* expr = expr_new(L, nterms);
    expr->constant = constant;
-   expr->term_count = nterms;
 
-   for (int i = 0; i < nterms; i++) {
+   for (int i = 0; i < nterms; ++i, ++expr->term_count) {
       const auto* term = get_term(L, i + 2);
       expr->terms[i].var = retain_unmanaged(term->var);
       expr->terms[i].coefficient = term->coefficient;
diff --git a/spec/expression_spec.lua b/spec/expression_spec.lua
new file mode 100644
index 0000000..89812ac
--- /dev/null
+++ b/spec/expression_spec.lua
@@ -0,0 +1,240 @@
+expose("module", function()
+   require("kiwi")
+end)
+
+describe("Expression", function()
+   local kiwi = require("kiwi")
+   local LUA_VERSION = tonumber(_VERSION:match("%d+%.%d+"))
+
+   it("construction", function()
+      local v = kiwi.Var("foo")
+      local v2 = kiwi.Var("bar")
+      local v3 = kiwi.Var("aux")
+      local e1 = kiwi.Expression(0, v * 1, v2 * 2, v3 * 3)
+      local e2 = kiwi.Expression(10, v * 1, v2 * 2, v3 * 3)
+
+      local constants = { 0, 10 }
+      for i, e in ipairs({ e1, e2 }) do
+         assert.equal(constants[i], e.constant)
+         local terms = e:terms()
+         assert.equal(3, #terms)
+         assert.equal(v, terms[1].var)
+         assert.equal(1.0, terms[1].coefficient)
+         assert.equal(v2, terms[2].var)
+         assert.equal(2.0, terms[2].coefficient)
+         assert.equal(v3, terms[3].var)
+         assert.equal(3.0, terms[3].coefficient)
+      end
+
+      if LUA_VERSION <= 5.2 then
+         assert.equal("1 foo + 2 bar + 3 aux + 10", tostring(e2))
+      else
+         assert.equal("1.0 foo + 2.0 bar + 3.0 aux + 10.0", tostring(e2))
+      end
+
+      assert.error(function()
+         kiwi.Expression(0, 0, v2 * 2, v3 * 3)
+      end)
+   end)
+
+   describe("method", function()
+      local v, t, e
+      before_each(function()
+         v = kiwi.Var("foo")
+         v:set(42)
+         t = kiwi.Term(v, 10)
+         e = t + 5
+      end)
+
+      it("has value", function()
+         v:set(42)
+         assert.equal(425, e:value())
+         v:set(87)
+         assert.equal(875, e:value())
+      end)
+
+      it("can be copied", function()
+         local e2 = e:copy()
+         assert.equal(e.constant, e2.constant)
+         local t1, t2 = e:terms(), e2:terms()
+         assert.equal(#t1, #t2)
+         for i = 1, #t1 do
+            assert.equal(t1[i].var, t2[i].var)
+            assert.equal(t1[i].coefficient, t2[i].coefficient)
+         end
+      end)
+
+      it("neg", function()
+         local neg = -e --[[@as kiwi.Expression]]
+         assert.True(kiwi.is_expression(neg))
+         local terms = neg:terms()
+         assert.equal(1, #terms)
+         assert.equal(v, terms[1].var)
+         assert.equal(-10.0, terms[1].coefficient)
+         assert.equal(-5, neg.constant)
+      end)
+
+      describe("bin op", function()
+         local v2, t2, e2
+         before_each(function()
+            v2 = kiwi.Var("bar")
+            t2 = kiwi.Term(v2)
+            e2 = v2 - 10
+         end)
+
+         it("mul", function()
+            for _, prod in ipairs({ e * 2.0, 2 * e }) do
+               assert.True(kiwi.is_expression(prod))
+               local terms = prod:terms()
+               assert.equal(1, #terms)
+               assert.equal(v, terms[1].var)
+               assert.equal(20.0, terms[1].coefficient)
+               assert.equal(10, prod.constant)
+            end
+
+            assert.error(function()
+               local _ = e * v
+            end)
+         end)
+
+         it("div", function()
+            local quot = e / 2.0
+            assert.True(kiwi.is_expression(quot))
+            local terms = quot:terms()
+            assert.equal(1, #terms)
+            assert.equal(v, terms[1].var)
+            assert.equal(5.0, terms[1].coefficient)
+            assert.equal(2.5, quot.constant)
+
+            assert.error(function()
+               local _ = e / v2
+            end)
+         end)
+
+         it("add", function()
+            for _, sum in ipairs({ e + 2.0, 2 + e }) do
+               assert.True(kiwi.is_expression(sum))
+               assert.equal(7.0, sum.constant)
+
+               local terms = sum:terms()
+               assert.equal(1, #terms)
+               assert.equal(10.0, terms[1].coefficient)
+               assert.equal(v, terms[1].var)
+            end
+
+            local sum = e + v2
+            assert.True(kiwi.is_expression(sum))
+            assert.equal(5, sum.constant)
+            local terms = sum:terms()
+            assert.equal(2, #terms)
+            assert.equal(v, terms[1].var)
+            assert.equal(10.0, terms[1].coefficient)
+            assert.equal(v2, terms[2].var)
+            assert.equal(1.0, terms[2].coefficient)
+
+            sum = e + t2
+            assert.True(kiwi.is_expression(sum))
+            assert.equal(5, sum.constant)
+            terms = sum:terms()
+            assert.equal(2, #terms)
+            assert.equal(v, terms[1].var)
+            assert.equal(10.0, terms[1].coefficient)
+            assert.equal(v2, terms[2].var)
+            assert.equal(1.0, terms[2].coefficient)
+
+            sum = e + e2
+            assert.True(kiwi.is_expression(sum))
+            assert.equal(-5, sum.constant)
+            terms = sum:terms()
+            assert.equal(2, #terms)
+            assert.equal(v, terms[1].var)
+            assert.equal(10.0, terms[1].coefficient)
+            assert.equal(v2, terms[2].var)
+            assert.equal(1.0, terms[2].coefficient)
+
+            assert.error(function()
+               local _ = t + "foo"
+            end)
+            assert.error(function()
+               local _ = t + {}
+            end)
+         end)
+
+         it("sub", function()
+            local constants = { 3, -3 }
+            for i, diff in ipairs({ e - 2.0, 2 - e }) do
+               local constant = constants[i]
+               assert.True(kiwi.is_expression(diff))
+               assert.equal(constant, diff.constant)
+
+               local terms = diff:terms()
+               assert.equal(1, #terms)
+               assert.equal(v, terms[1].var)
+               assert.equal(constant < 0 and -10.0 or 10.0, terms[1].coefficient)
+            end
+
+            local diff = e - v2
+            assert.True(kiwi.is_expression(diff))
+            assert.equal(5, diff.constant)
+            local terms = diff:terms()
+            assert.equal(2, #terms)
+            assert.equal(v, terms[1].var)
+            assert.equal(10.0, terms[1].coefficient)
+            assert.equal(v2, terms[2].var)
+            assert.equal(-1.0, terms[2].coefficient)
+
+            diff = e - t2
+            assert.True(kiwi.is_expression(diff))
+            assert.equal(5, diff.constant)
+            terms = diff:terms()
+            assert.equal(2, #terms)
+            assert.equal(v, terms[1].var)
+            assert.equal(10.0, terms[1].coefficient)
+            assert.equal(v2, terms[2].var)
+            assert.equal(-1.0, terms[2].coefficient)
+
+            diff = e - e2
+            assert.True(kiwi.is_expression(diff))
+            assert.equal(15, diff.constant)
+            terms = diff:terms()
+            assert.equal(2, #terms)
+            assert.equal(v, terms[1].var)
+            assert.equal(10.0, terms[1].coefficient)
+            assert.equal(v2, terms[2].var)
+            assert.equal(-1.0, terms[2].coefficient)
+
+            assert.error(function()
+               local _ = e - "foo"
+            end)
+            assert.error(function()
+               local _ = e - {}
+            end)
+         end)
+
+         it("constraint expr op expr", function()
+            local ops = { "LE", "EQ", "GE" }
+            for i, meth in ipairs({ "le", "eq", "ge" }) do
+               local c = e[meth](e, e2)
+               assert.True(kiwi.is_constraint(c))
+
+               local expr = c:expression()
+               local terms = expr:terms()
+               assert.equal(2, #terms)
+
+               -- order can be randomized due to use of map
+               if terms[1].var ~= v then
+                  terms[1], terms[2] = terms[2], terms[1]
+               end
+               assert.equal(v, terms[1].var)
+               assert.equal(10.0, terms[1].coefficient)
+               assert.equal(v2, terms[2].var)
+               assert.equal(-1.0, terms[2].coefficient)
+
+               assert.equal(15, expr.constant)
+               assert.equal(ops[i], c:op())
+               assert.equal(kiwi.strength.REQUIRED, c:strength())
+            end
+         end)
+      end)
+   end)
+end)
diff --git a/spec/term_spec.lua b/spec/term_spec.lua
new file mode 100644
index 0000000..5919de2
--- /dev/null
+++ b/spec/term_spec.lua
@@ -0,0 +1,245 @@
+expose("module", function()
+   require("kiwi")
+end)
+
+describe("Term", function()
+   local kiwi = require("kiwi")
+   local LUA_VERSION = tonumber(_VERSION:match("%d+%.%d+"))
+
+   it("construction", function()
+      local v = kiwi.Var("foo")
+      local t = kiwi.Term(v)
+      assert.equal(v, t.var)
+      assert.equal(1.0, t.coefficient)
+
+      t = kiwi.Term(v, 100)
+      assert.equal(v, t.var)
+      assert.equal(100, t.coefficient)
+
+      if LUA_VERSION <= 5.2 then
+         assert.equal("100 foo", tostring(t))
+      else
+         assert.equal("100.0 foo", tostring(t))
+      end
+
+      assert.error(function()
+         kiwi.Term("")
+      end)
+   end)
+
+   describe("method", function()
+      local v, v2, t, t2
+
+      before_each(function()
+         v = kiwi.Var("foo")
+         t = kiwi.Term(v, 10)
+      end)
+
+      it("has value", function()
+         v:set(42)
+         assert.equal(420, t:value())
+         v:set(87)
+         assert.equal(870, t:value())
+      end)
+
+      it("has toexpr", function()
+         local e = t:toexpr()
+         assert.True(kiwi.is_expression(e))
+         assert.equal(0, e.constant)
+         local terms = e:terms()
+         assert.equal(1, #terms)
+         assert.equal(v, terms[1].var)
+         assert.equal(10.0, terms[1].coefficient)
+      end)
+
+      it("neg", function()
+         local neg = -t --[[@as kiwi.Term]]
+         assert.True(kiwi.is_term(neg))
+         assert.equal(v, neg.var)
+         assert.equal(-10, neg.coefficient)
+      end)
+
+      describe("bin op", function()
+         before_each(function()
+            v2 = kiwi.Var("bar")
+            t2 = kiwi.Term(v2)
+         end)
+
+         it("mul", function()
+            for _, prod in ipairs({ t * 2.0, 2 * t }) do
+               assert.True(kiwi.is_term(prod))
+               assert.equal(v, prod.var)
+               assert.equal(20, prod.coefficient)
+            end
+
+            assert.error(function()
+               local _ = t * v
+            end)
+         end)
+
+         it("div", function()
+            local quot = t / 2.0
+            assert.True(kiwi.is_term(quot))
+            assert.equal(v, quot.var)
+            assert.equal(5.0, quot.coefficient)
+
+            assert.error(function()
+               local _ = v / v2
+            end)
+         end)
+
+         it("add", function()
+            for _, sum in ipairs({ t + 2.0, 2 + t }) do
+               assert.True(kiwi.is_expression(sum))
+               assert.equal(2.0, sum.constant)
+
+               local terms = sum:terms()
+               assert.equal(1, #terms)
+               assert.equal(10.0, terms[1].coefficient)
+               assert.equal(v, terms[1].var)
+            end
+
+            local sum = t + v2
+            assert.True(kiwi.is_expression(sum))
+            assert.equal(0, sum.constant)
+            local terms = sum:terms()
+            assert.equal(2, #terms)
+            assert.equal(v, terms[1].var)
+            assert.equal(10.0, terms[1].coefficient)
+            assert.equal(v2, terms[2].var)
+            assert.equal(1.0, terms[2].coefficient)
+
+            sum = t + t2
+            assert.True(kiwi.is_expression(sum))
+            assert.equal(0, sum.constant)
+            terms = sum:terms()
+            assert.equal(2, #terms)
+            assert.equal(v, terms[1].var)
+            assert.equal(10.0, terms[1].coefficient)
+            assert.equal(v2, terms[2].var)
+            assert.equal(1.0, terms[2].coefficient)
+
+            local t3 = kiwi.Term(v2, 20)
+            sum = t3 + sum
+            assert.True(kiwi.is_expression(sum))
+            assert.equal(0, sum.constant)
+            terms = sum:terms()
+            assert.equal(3, #terms)
+            assert.equal(v, terms[1].var)
+            assert.equal(10.0, terms[1].coefficient)
+            assert.equal(v2, terms[2].var)
+            assert.equal(1.0, terms[2].coefficient)
+            assert.equal(v2, terms[3].var)
+            assert.equal(20.0, terms[3].coefficient)
+
+            assert.error(function()
+               local _ = t + "foo"
+            end)
+            assert.error(function()
+               local _ = t + {}
+            end)
+         end)
+
+         it("sub", function()
+            local constants = { -2, 2 }
+            for i, diff in ipairs({ t - 2.0, 2 - t }) do
+               local constant = constants[i]
+               assert.True(kiwi.is_expression(diff))
+               assert.equal(constant, diff.constant)
+
+               local terms = diff:terms()
+               assert.equal(1, #terms)
+               assert.equal(v, terms[1].var)
+               assert.equal(constant < 0 and 10.0 or -10.0, terms[1].coefficient)
+            end
+
+            local diff = t - v2
+            assert.True(kiwi.is_expression(diff))
+            assert.equal(0, diff.constant)
+            local terms = diff:terms()
+            assert.equal(2, #terms)
+            assert.equal(v, terms[1].var)
+            assert.equal(10.0, terms[1].coefficient)
+            assert.equal(v2, terms[2].var)
+            assert.equal(-1.0, terms[2].coefficient)
+
+            diff = t - t2
+            assert.True(kiwi.is_expression(diff))
+            assert.equal(0, diff.constant)
+            terms = diff:terms()
+            assert.equal(2, #terms)
+            assert.equal(v, terms[1].var)
+            assert.equal(10.0, terms[1].coefficient)
+            assert.equal(v2, terms[2].var)
+            assert.equal(-1.0, terms[2].coefficient)
+
+            local t3 = kiwi.Term(v2, 20)
+            diff = t3 - diff
+            assert.True(kiwi.is_expression(diff))
+            assert.equal(0, diff.constant)
+            terms = diff:terms()
+            assert.equal(3, #terms)
+            assert.equal(v, terms[1].var)
+            assert.equal(-10.0, terms[1].coefficient)
+            assert.equal(v2, terms[2].var)
+            assert.equal(1.0, terms[2].coefficient)
+            assert.equal(v2, terms[3].var)
+            assert.equal(20.0, terms[3].coefficient)
+
+            assert.error(function()
+               local _ = t - "foo"
+            end)
+            assert.error(function()
+               local _ = t - {}
+            end)
+         end)
+
+         it("constraint term op expr", function()
+            local ops = { "LE", "EQ", "GE" }
+            for i, meth in ipairs({ "le", "eq", "ge" }) do
+               local c = t[meth](t, v2 + 1)
+               assert.True(kiwi.is_constraint(c))
+
+               local e = c:expression()
+               local terms = e:terms()
+               assert.equal(2, #terms)
+
+               -- order can be randomized due to use of map
+               if terms[1].var ~= v then
+                  terms[1], terms[2] = terms[2], terms[1]
+               end
+               assert.equal(v, terms[1].var)
+               assert.equal(10.0, terms[1].coefficient)
+               assert.equal(v2, terms[2].var)
+               assert.equal(-1.0, terms[2].coefficient)
+
+               assert.equal(-1, e.constant)
+               assert.equal(ops[i], c:op())
+               assert.equal(kiwi.strength.REQUIRED, c:strength())
+            end
+         end)
+
+         it("constraint term op term", function()
+            for i, meth in ipairs({ "le", "eq", "ge" }) do
+               local c = t[meth](t, t2)
+               assert.True(kiwi.is_constraint(c))
+
+               local e = c:expression()
+               local terms = e:terms()
+               assert.equal(2, #terms)
+
+               -- order can be randomized due to use of map
+               if terms[1].var ~= v then
+                  terms[1], terms[2] = terms[2], terms[1]
+               end
+               assert.equal(v, terms[1].var)
+               assert.equal(10.0, terms[1].coefficient)
+               assert.equal(v2, terms[2].var)
+               assert.equal(-1.0, terms[2].coefficient)
+
+               assert.equal(0, e.constant)
+            end
+         end)
+      end)
+   end)
+end)
diff --git a/spec/var_spec.lua b/spec/var_spec.lua
index 3bd693a..efc830f 100644
--- a/spec/var_spec.lua
+++ b/spec/var_spec.lua
@@ -127,6 +127,8 @@ describe("Var", function()
             assert.equal(v2, terms[2].var)
             assert.equal(-1.0, terms[2].coefficient)
 
+            -- TODO: terms and expressions
+
             assert.error(function()
                local _ = v - "foo"
             end)