Operations must be functions

Functions and operations must not be two different things: Technically, operations are functions. By adopting the proper definition of operation, the design and the low-level implementation of the so-far-called "developer functions" and "operations" can be merged into the notion of "functions".

As a consequence, functions should not be implemented as operands and the notion of functions should be implemented at booleano.nodes.functions. Then operations at booleano.nodes.operations would extend this class.