diff --git a/build/error.go b/build/error.go
index 7a03308..1da00cb 100644
--- a/build/error.go
+++ b/build/error.go
@@ -40,6 +40,7 @@ func (e *ErrorList) Add(err Error) {
 	*e = append(*e, err)
 }
 
+// AddAt appends an error at position p.
 func (e *ErrorList) AddAt(p src.Position, err error) {
 	e.Add(Error{p, err})
 }