[drallison]

Here's another way to do this in Python.

   lets =  [chr(x) for x in range( ord('a'),ord('z')+1)]

   def blankexpand( templates ):
       expansion = []
       for t in templates:
        expansion = expansion + [t.replace('_',let,1) for let in lets]
       return expansion


   def expand( template ):
        blanktiles = template.count('_')
        if blanktiles == 0:
                print 'nothing to expand'
        elif blanktiles == 1:
                print blankexpand( [template] )
        elif blanktiles == 2:
                print blankexpand( blankexpand( [template] ) )
        else :
                print 'too many blank tiles'
        print


   expand( 'abce' )
   expand( 'a_')
   expand( 'h_ll_')
   expand( '___' )