Remove quotation from strings.

builder.py

@@ -44,10 +44,14 @@ t_ATOM = '[a-zA-Z][_0-9a-zA-Z]*'
 t_COMMENT = '\#[^\n]*'
 t_COMMA = ','
 t_DOT = '\.'
-t_STRING = "'[^']*'"
 
 t_ignore = ' \t'
 
+def t_STRING(t):
+    "'[^']*'"
+    t.value = t.value[1:-1]
+    return t
+
 def t_EOL(t):
     r'\n'
     t.lexer.lineno += 1