diff --git a/Makefile b/Makefile
index 87236e3..fa8c069 100644
--- a/Makefile
+++ b/Makefile
@@ -5,14 +5,14 @@ CFLAGS += -Wall -I$(SRCDIR)/kiwi
 LIBFLAG := -shared
 LIB_EXT := so
 
-ifeq ($(findstring gcc, $(CC)), gcc)
-  CXX := $(subst gcc, g++, $(CC))
+ifeq ($(findstring gcc,$(CC)),gcc)
+  CXX := $(subst gcc,g++,$(CC))
   CXXFLAGS += -std=c++14
   ifneq ($(SANITIZE),)
     CFLAGS += -fsanitize=undefined -fsanitize=address
   endif
 else
-ifeq ($(CC), clang)
+ifeq ($(CC),clang)
   CXX := clang++
   CXXFLAGS += -std=c++14
   ifneq ($(SANITIZE),)
diff --git a/kiwi.lua b/kiwi.lua
index 9a60ab0..3a9e674 100644
--- a/kiwi.lua
+++ b/kiwi.lua
@@ -1,6 +1,15 @@
-local kiwi = {}
+-- kiwi.lua - LuaJIT FFI bindings with C API fallback to kiwi constraint solver.
+
+local ffi
+do
+   local ffi_loader = package.preload["ffi"]
+   if ffi_loader == nil then
+      return require("ckiwi")
+   end
+   ffi = ffi_loader() --[[@as ffilib]]
+end
 
-local ffi = require("ffi")
+local kiwi = {}
 
 local ckiwi
 do
@@ -101,7 +110,7 @@ local ffi_copy, ffi_gc, ffi_istype, ffi_new, ffi_string =
 local concat = table.concat
 local has_table_new, new_tab = pcall(require, "table.new")
 if not has_table_new or type(new_tab) ~= "function" then
-   new_tab = function()
+   new_tab = function(_, _)
       return {}
    end
 end
@@ -532,7 +541,7 @@ end
 do
    --- Expressions are a sum of terms with an added constant.
    ---@class kiwi.Expression: ffi.cdata*
-   ---@overload fun(terms: kiwi.Term[], constant: number?): kiwi.Expression
+   ---@overload fun(constant: number, ...: kiwi.Term): kiwi.Expression
    ---@field constant number
    ---@field package term_count number
    ---@field package terms_ ffi.cdata*
@@ -610,7 +619,8 @@ do
    function Expression_cls:value()
       local sum = self.constant
       for i = 0, self.term_count - 1 do
-         sum = sum + self.terms_[i]:value()
+         local t = self.terms_[i]
+         sum = sum + t.var:value() * t.coefficient
       end
       return sum
    end
@@ -636,17 +646,16 @@ do
       __index = Expression_cls,
    }
 
-   function Expression_mt.__new(T, terms, constant)
-      local term_count = terms and #terms or 0
+   function Expression_mt.__new(T, constant, ...)
+      local term_count = select("#", ...)
       local e = ffi_gc(ffi_new(T, term_count), ckiwi.kiwi_expression_del_vars) --[[@as kiwi.Expression]]
       e.term_count = term_count
-      e.constant = constant or 0.0
-      if terms then
-         for i, t in ipairs(terms) do
-            local dt = e.terms_[i - 1] --[[@as kiwi.Term]]
-            dt.var = ckiwi.kiwi_var_clone(t.var)
-            dt.coefficient = t.coefficient
-         end
+      e.constant = constant
+      for i = 1, term_count do
+         local t = select(i, ...)
+         local dt = e.terms_[i - 1] --[[@as kiwi.Term]]
+         dt.var = ckiwi.kiwi_var_clone(t.var)
+         dt.coefficient = t.coefficient
       end
       return e
    end
@@ -729,7 +738,7 @@ do
    ---@return kiwi.Expression
    ---@nodiscard
    function Constraint_cls:expression()
-      local SZ = 8
+      local SZ = 7 -- 2**7 bytes on x64
       local expr = ffi_new(Expression, SZ) --[[@as kiwi.Expression]]
       local n = ckiwi.kiwi_constraint_expression(self, expr, SZ)
       if n > SZ then
@@ -865,7 +874,7 @@ do
    --- Produce a custom error raise mask
    --- Error kinds specified in the mask will not cause a lua
    --- error to be raised.
-   ---@param kinds (kiwi.ErrKind|number)[]
+   ---@param kinds (kiwi.ErrKind|integer)[]
    ---@param invert boolean?
    ---@return integer
    function kiwi.error_mask(kinds, invert)
@@ -948,7 +957,7 @@ do
    end
    ---@class kiwi.Solver: ffi.cdata*
    ---@field package error_mask_ integer
-   ---@overload fun(error_mask: (integer|(kiwi.ErrKind|number)[] )?): kiwi.Solver
+   ---@overload fun(error_mask: (integer|(kiwi.ErrKind|integer)[] )?): kiwi.Solver
    local Solver_cls = {
       --- Test whether a constraint is in the solver.
       ---@type fun(self: kiwi.Solver, constraint: kiwi.Constraint): boolean
@@ -978,7 +987,7 @@ do
    }
 
    --- Sets the error mask for the solver.
-   ---@param mask integer|(kiwi.ErrKind|number)[] the mask value or an array of kinds
+   ---@param mask integer|(kiwi.ErrKind|integer)[] the mask value or an array of kinds
    ---@param invert boolean? whether to invert the mask if an array was passed for mask
    function Solver_cls:set_error_mask(mask, invert)
       if type(mask) == "table" then
diff --git a/spec/constraint_spec.lua b/spec/constraint_spec.lua
index 137a228..61144c0 100644
--- a/spec/constraint_spec.lua
+++ b/spec/constraint_spec.lua
@@ -42,7 +42,7 @@ describe("Constraint", function()
          c = kiwi.Constraint(lhs / 2, nil, "LE", kiwi.strength.MEDIUM)
          assert.equal("0.5 foo + 0.5 <= 0 | medium", tostring(c))
 
-         c = kiwi.Constraint(lhs, kiwi.Expression(nil, 3), "GE", kiwi.strength.WEAK)
+         c = kiwi.Constraint(lhs, kiwi.Expression(3), "GE", kiwi.strength.WEAK)
          assert.equal("1 foo + -2 >= 0 | weak", tostring(c))
       end)
 
@@ -68,7 +68,7 @@ describe("Constraint", function()
       end)
       it("combines lhs and rhs", function()
          local v2 = kiwi.Var("bar")
-         local rhs = kiwi.Expression({ 5 * v2, 3 * v }, 3)
+         local rhs = kiwi.Expression(3, 5 * v2, 3 * v)
          local c = kiwi.Constraint(lhs, rhs)
 
          local e = c:expression()