diff --git a/syntax/ats.vim b/syntax/ats.vim
index 40ca895..d74fa27 100644
--- a/syntax/ats.vim
+++ b/syntax/ats.vim
@@ -77,7 +77,7 @@ syn keyword atsStatement val and fnx fun fn llam lam fix rec var of assume castf
 syn keyword atsStatement begin end
 syn keyword atsTypedef typedef sortdef viewtypedef vtypedef viewdef
 syn keyword atsStructure datatype datavtype abstype dataviewtype dataprop dataview
-syn keyword atsExternal extern implement
+syn keyword atsExternal extern implement primplmnt
 
 syn keyword atsKeyword symintr overload with
 syn keyword atsKeyword prval praxi datasort