Question: trick in eval: procedural substitution

After understanding how one part of code works i also see that it uses trick of eval (namely "procedural substitution"):

restart;
r:=f(g(u), h(w)):
eval(r, [g=proc() ll; end proc]):

gets full substitution with arguments

and

restart;
r:=f(g(u), h(w)):
eval(r, [g=ll]):

do simple one 'by name'.

Where can i read about that?

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