// Elucidate the exception.
throw new NullPointerException("type == null");
}
int idx = stackPtr - n - 1;
TypeBearer orig = stack[idx];
if ((orig == null) ||
(orig.getType().getCategory() != type.getType().getCategory())) {
throwSimException("incompatible substitution: " +
stackElementString(orig) + " -> " +
stackElementString(type));
}