From 2a71b3f580d720cf8afd6172e84d1296c57f8c75 Mon Sep 17 00:00:00 2001
From: Luc Grosheintz <luc.grosheintz@gmail.com>
Date: Thu, 10 Oct 2024 08:58:14 +0200
Subject: [PATCH 1/4] Improve documentation.

---
 src/language/nmodl.yaml | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/language/nmodl.yaml b/src/language/nmodl.yaml
index 0f76b01a3..de598cec3 100644
--- a/src/language/nmodl.yaml
+++ b/src/language/nmodl.yaml
@@ -1140,15 +1140,15 @@
                             type: Expression
 
                   - NonLinEquation:
-                      brief: "TODO"
+                      brief: "One equation in a system of equations that collectively make a NONLINEAR block."
                       nmodl: "~ "
                       members:
                         - lhs:
-                            brief: "TODO"
+                            brief: "Left-hand-side of the equation."
                             type: Expression
                             suffix: {value: " = "}
                         - rhs:
-                            brief: "TODO"
+                            brief: "Right-hand-side of the equation."
                             type: Expression
 
                   - LinEquation:

From ac7b0da7523d7e506e72139226e245c8dcc4be2c Mon Sep 17 00:00:00 2001
From: Luc Grosheintz <luc.grosheintz@gmail.com>
Date: Thu, 10 Oct 2024 11:33:21 +0200
Subject: [PATCH 2/4] A `NONLINEAR` block requires `sympy --analytic`.

---
 src/main.cpp | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/main.cpp b/src/main.cpp
index 436ec2c4e..ba9de4a19 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -516,6 +516,8 @@ int run_nmodl(int argc, const char* argv[]) {
 
             enable_sympy(solver_exists(*ast, "derivimplicit"), "'SOLVE ... METHOD derivimplicit'");
             enable_sympy(node_exists(*ast, ast::AstNodeType::LINEAR_BLOCK), "'LINEAR' block");
+            enable_sympy(node_exists(*ast, ast::AstNodeType::NON_LINEAR_BLOCK),
+                         "'NONLINEAR' block");
             enable_sympy(solver_exists(*ast, "sparse"), "'SOLVE ... METHOD sparse'");
         }
 

From 36eeee26843dff57d2326a166324b1a17eff0a5b Mon Sep 17 00:00:00 2001
From: Luc Grosheintz <luc.grosheintz@gmail.com>
Date: Thu, 10 Oct 2024 11:49:24 +0200
Subject: [PATCH 3/4] Add trivial test.

---
 test/usecases/nonlinear/nonlin.mod     | 21 +++++++++++++++++++++
 test/usecases/nonlinear/test_nonlin.py | 15 +++++++++++++++
 2 files changed, 36 insertions(+)
 create mode 100644 test/usecases/nonlinear/nonlin.mod
 create mode 100644 test/usecases/nonlinear/test_nonlin.py

diff --git a/test/usecases/nonlinear/nonlin.mod b/test/usecases/nonlinear/nonlin.mod
new file mode 100644
index 000000000..ec0bed30f
--- /dev/null
+++ b/test/usecases/nonlinear/nonlin.mod
@@ -0,0 +1,21 @@
+NEURON {
+    SUFFIX nonlin
+}
+
+STATE { x }
+
+FUNCTION solve() {
+    : Iterative solvers need a starting guess.
+    x = 1.0
+    SOLVE eq
+
+    solve = x
+}
+
+NONLINEAR eq {
+    ~ x*x = 4.0
+}
+
+FUNCTION residual(x) {
+    residual = x - 2.0
+}
diff --git a/test/usecases/nonlinear/test_nonlin.py b/test/usecases/nonlinear/test_nonlin.py
new file mode 100644
index 000000000..eef75edbd
--- /dev/null
+++ b/test/usecases/nonlinear/test_nonlin.py
@@ -0,0 +1,15 @@
+from neuron import h, gui
+
+
+def test_nonlin():
+    s = h.Section()
+    s.insert("nonlin")
+    inst = s(0.5).nonlin
+
+    x = inst.solve()
+    error = inst.residual(x)
+    assert abs(error) < 1e-9, f"{x = }, {error = }"
+
+
+if __name__ == "__main__":
+    test_nonlin()

From d0b0a38b2988133024b896828629f72983e766a6 Mon Sep 17 00:00:00 2001
From: Luc Grosheintz <luc.grosheintz@gmail.com>
Date: Mon, 14 Oct 2024 13:25:49 +0200
Subject: [PATCH 4/4] Register `nonlinear` tests.

---
 test/usecases/CMakeLists.txt | 1 +
 1 file changed, 1 insertion(+)

diff --git a/test/usecases/CMakeLists.txt b/test/usecases/CMakeLists.txt
index feaa81ef7..4d0700fbc 100644
--- a/test/usecases/CMakeLists.txt
+++ b/test/usecases/CMakeLists.txt
@@ -17,6 +17,7 @@ set(NMODL_USECASE_DIRS
     net_receive
     net_send
     neuron_variables
+    nonlinear
     nonspecific_current
     parameter
     point_process