nand method
Combines this spec with another using NAND logic (NOT AND).
Returns a new spec that is satisfied when it is NOT the case that both
this spec and other are satisfied — i.e., at least one must fail.
Equivalent to calling and followed by toNegated.
Prefer someSpec.and(other).toNegated() when readability matters.
Use nand as a convenience when the negated-AND intent is central
to the business rule.
Example:
// True unless both conditions hold simultaneously
final notBothAdmin = IsAdmin().nand(IsSuperuser());
Implementation
Spec<T> nand(Spec<T> other) => _Nand(this, other);