diff --git a/maths/fast.hpp b/maths/fast.hpp
index f726f072..e9dfe108 100644
--- a/maths/fast.hpp
+++ b/maths/fast.hpp
@@ -12,6 +12,8 @@
 #include "../debug/assert.hpp"
 
 namespace cruft::maths::fast {
+    // computes: a + t * b + t^2 * c + t^3 * d, but hopefully a little more
+    // efficiently than typing out the above directly.
     constexpr float
     estrin (float t, float a, float b, float c, float d)
     {