Method swap12

chapter21/src/main/java/com/seaofnodes/simple/node/Node.java:650–654

()

    // Swap inputs without letting either input go dead during the swap.
    public Node swap12() {
        unlock();               // Hash is order dependent
        _inputs.swap(1,2);
        return this;
    }

    /**
     * Does this node contain all constants?  Ignores in(0), as is usually