One attribute instead of two in Map, Filter and Reduce

In Filter, Map and Reduce we have a either the lambda_ or the function attribute defined:

lambda_ = Lambda | function = [FunctionDefinition]

But in the interpreter, both are used in the same way:

func_def = self.lambda_ if self.lambda_ else self.function

The rule should then look like this:

function = Lambda | function = [FunctionDefinition]

to just use self.function.