FunctionNode represents a node that corresponds to a specific function in the requirement graph.
It holds a reference to a FunctionDefinition and can have child nodes
that represent rules, functions, or combinations that depend on the output of this function.
FunctionNode represents a node that corresponds to a specific function in the requirement graph. It holds a reference to a FunctionDefinition and can have child nodes that represent rules, functions, or combinations that depend on the output of this function.