Type Substitutability in p6i

• Type and interface corresponds 1-1 in p6i.
• Type A can substitute type B if the corresponding interface A' is (possibly indirect-) subinterface of B'.
  • informally ... B has same or more detailed state distinction and narrower synchronization condition or more constraints compared to A.
• This does not (of courese!) assure TRUE (formally provable?) substitutability, but will be useful enough as programmer's tool.
  • c.f. relation of Java interface and "implements" relation.