diff --git a/src/SourceSpan.hs b/src/SourceSpan.hs
index a81c63a21..e486b31ca 100644
--- a/src/SourceSpan.hs
+++ b/src/SourceSpan.hs
@@ -1,5 +1,6 @@
 -- |
 -- Source position and span information
+-- Mostly taken from purescript's SourcePos definition.
 --
 module SourceSpan where