patch 9.0.0012: signature files not detected properly

Problem:    Signature files not detected properly.
Solution:   Add a function to better detect signature files. (Doug Kearns)

runtime/autoload/dist/ft.vim

@@ -459,7 +459,7 @@ export def FTmm()
   setf nroff
 enddef
 
-# Returns true if file content looks like LambdaProlog
+# Returns true if file content looks like LambdaProlog module
 def IsLProlog(): bool
   # skip apparent comments and blank lines, what looks like 
   # LambdaProlog comment may be RAPID header
@@ -848,6 +848,27 @@ export def FTperl(): number
   return 0
 enddef
 
+# LambdaProlog and Standard ML signature files
+export def FTsig()
+  if exists("g:filetype_sig")
+    exe "setf " .. g:filetype_sig
+    return
+  endif
+
+  var lprolog_comment = '^\s*\%(/\*\|%\)'
+  var lprolog_keyword = '^\s*sig\s\+\a'
+  var sml_comment = '^\s*(\*'
+  var sml_keyword = '^\s*\%(signature\|structure\)\s\+\a'
+
+  var line = getline(nextnonblank(1))
+
+  if line =~ lprolog_comment || line =~# lprolog_keyword
+    setf lprolog
+  elseif line =~ sml_comment || line =~# sml_keyword
+    setf sml
+  endif
+enddef
+
 export def FTsys()
   if exists("g:filetype_sys")
     exe "setf " .. g:filetype_sys