!7

replace Var with ValuelessVar.
replace VarMap with Map<Var, O>.

abstract sclass Prop implements IStatement, IFieldsToList {}

static withToList record $HappensBefore$<A, B>(A event1, B event2) extends Prop {}
static withToList record $HappensAtDay$<A, B>(A event, B y) extends Prop {}
static withToList record $LessThan$<A, B>(A x, B y) extends Prop {}
static withToList record $BiggerThanOrEqualTo$<A, B>(A x, B y) extends Prop {}
static withToList record $AnyIntBiggerThan(O x) {}
static withToList record $is$(O a, O b) {}

// full define $LessThan(int x, int y) = x < y;
static Bool checkProposition(Prop p) {
  if p is $LessThan$(int x, int y) { ret x < y; }
  if p is $BiggerThanOrEqualTo$(int x, int y) {
    ret not(checkProposition($LessThan$(x, y)));
  }
  null;
}

static VarMap fillVariablesInProposition(Prop p) {
  if p is $LessThan$(int x, Var y) {
    $AnyIntBiggerThan
  }
}

p-exp {
  assertEqualsVerbose(true, checkProposition($LessThan$(1, 5)));
  assertEqualsVerbose(false, checkProposition($LessThan$(5, 1)));
  assertEqualsVerbose(null, checkProposition($LessThan$("what", "ever")));
  assertEqualsVerbose(false, checkProposition($BiggerThanOrEqualTo$(1, 5)));
  assertEqualsVerbose(true, checkProposition($BiggerThanOrEqualTo$(5, 1)));
  assertEqualsVerbose(null, checkProposition($BiggerThanOrEqualTo$("what", "ever")));
}

Began life as a copy of #1025427