code: public final void addConstraint(@NonNull final Constraint<CharSequence> constraint) {
          Condition.INSTANCE.ensureNotNull(constraint, "The constraint may not be null");
  
          if (!constraints.contains(constraint)) {
              constraints.add(constraint);
              verifyPasswordStrength();
          }
      }
docs: Adds a new constraint, which should be used to verify the password strength.
  
  @param constraint
  The constraint, which should be added, as an instance of the type {@link Constraint}.
  The constraint may not be null