return update(cond,newThen,newElse);
}
@Override
public Expression substitute(IPersistentMap bindings) {
Expression ncond=cond.substitute(bindings);
if (ncond==null) return null;
Expression nthen=doThen.substitute(bindings);
if (nthen==null) return null;
Expression nelse=doElse.substitute(bindings);
if (nelse==null) return null;
return update(ncond,nthen,nelse);
}