parameters: (self,name)
docstring: Retrieves a named expression
docstring_summary: Retrieves a named expression
docstring_tokens: Retrieves a named expression
function: def expr(self,name):
          """Retrieves a named expression"""
          try:
              return self.expressions[name]
          except KeyError:
              pass
          raise KeyError("Invalid expression name "+name)
function_tokens: def expr ( self , name ) : try : return self . expressions [ name ] except KeyError : pass raise KeyError ( "Invalid expression name " + name )