Moreover, what is the exact relationship between a valid index and a pseudo index? The manual has its inconsistency:
The manual says that a valid index refers to a real stack postition, whereas a pseudo index does not. So by definition they are totally different things. No pseudo index can be a valid index.
But, in section 4.3:
An acceptable index can be any valid index, including the pseudo-indices, but it also can be any positive index after the stack top within the space allocated for the stack, that is, indices up to the stack size.
The word "including" seems to indicate that pseudo-indices are included in valid index, which contradicts the definition.