public Exp instReplace(Exp olde, Exp newe) {
if (this == olde) return newe;
Exp e;
for (int i = 0; i < exps.size(); i++) {
e = exps.get(i);
exps.set(i, e.instReplace(olde, newe));
e = exps.get(i);
if (e == null) return null;
}
return this;
}
