syn match factorForget /\<FORGET:\s\+\S\+\>/
syn match factorMixin /\<MIXIN:\s\+\S\+\>/
syn match factorInstance /\<INSTANCE:\s\+\S\+\s\+\S\+\>/
-syn match factorHook /\<HOOK:\s\+\S\+\s\+\S\+\>/
+syn match factorHook /\<HOOK:\s\+\S\+\s\+\S\+\>/ nextgroup=factorStackEffect skipwhite skipempty
syn match factorMain /\<MAIN:\s\+\S\+\>/
syn match factorConstructor /\<C:\s\+\S\+\s\+\S\+\>/
syn match factorAlien /\<ALIEN:\s\+[0-9a-fA-F]\([0-9a-fA-F,]*[0-9a-fA-F]\)\?\>/