MX changes input structure

[Bart Bogaerts]

Specifically, the vocabulary of the input structure:

vocabulary V_part {
    type X
}

structure  S : V_part {}

vocabulary V {
    extern vocabulary V_part
    type Y
}

theory T : V {}

procedure main() {
    S_original = clone(S)
    print(equal(S, S_original))        // --> false
    print(S)
    print(S_original)
}   

Since we have to make a copy of the input anyway (to be used as output), we might as well do it a bit earlier.

MX (internally) should get a const structure in. But @JoD probably removed constness! ;-)