Function signedMembership

src/compute-engine/library/sets.ts:76–86

* Three-valued membership for a set defined by a base type together with a * sign predicate (e.g. the negative reals). `sign` is the relevant three-valued * sign property of `x` (e.g. `x.isNegative`).

(
  x: Expression,
  baseType: Type,
  sign: boolean | undefined
)

// compactification, so the unsigned `~oo` of the Riemann sphere is not a
// member of any of these three. The extended COMPLEX plane is the one that
// takes it, and it takes every infinite magnitude with it.
//
// The infinities are spelled as the two value types `+oo | -oo` — the
// signed pair {+∞, −∞}, which is also what the retired one-word name
// `non_finite_number` normalizes to on input. An operand typed with the
// pair (`Ln(0)`, a symbol declared with the retired spelling) IS a subtype
// of `real | +oo | -oo`, so its membership decides. The pair excludes both
// `~oo` and NaN, so the two-point claim above is unaffected.
const EXTENDED_RATIONAL_TYPE = parseType('rational | +oo | -oo');
const EXTENDED_INTEGER_TYPE = parseType('integer | +oo | -oo');
const EXTENDED_COMPLEX_TYPE = parseType('complex | infinity');

/**
 * Build the `subsetOf` handler of a number set that contains EVERY value of