simplify an expression with given rules which are functional relations

Question

In maple one can simplify expression using self-defined rules, like for example

> simplify(x*y,{x=y+1})
y^2+y

In contrast, what I need is the ability to pass rules which are in functional form, i.e. something like

> wantedcommand(f(a)*f(b), f(x)=g(x+3))
g(a+3)*g(b+3)

Does somebody know how to achieve this?

Answer 1

applyrule(f(x::anything)= g(x+3), f(a)*f(b));