Definition of "substitutability":

From Dan Smith:
One (might be an algorithm or an optimization) can replace x with y without noticing the difference.

From John Rose:
What Dan said. Non-value references are s== iff they are acmp==. Value instances are s== iff they are component-wise s==. Integral types are s== iff they are op==. Float types are s== iff their raw-bits are op==. This is what amounts to bitwise strict image equality mostly except for invisible references (for types which fail to flatten fully) and also NaNs are folded to one.