M(input) =
Vresult = V(input),
// try for V
[tree1, residue1] =
Vresult,
residue1 == [] ?
Vresult
// use M -> V
: failed(tree1) ? Vresult
// failure
: first(residue1) == '*' ?
( [tree2,
residue2] = M(rest(residue1)),
// try M -> V * M
failed(tree2) ?
Vresult
// use M -> V only
:
[mkTree('*', tree1, tree2), residue2] // use
M -> V + M
)
: Vresult;
// use M -> V