abs.erl:16: The pattern 
          'true' can never match the type 
          'false'
abs.erl:27: The pattern 
          'true' can never match the type 
          'false'
abs.erl:37: The pattern 
          'true' can never match the type 
          'false'
abs.erl:49: The pattern 
          'true' can never match the type 
          'false'