What the mind casts aside as worthless is exactly what its whole peace rests upon. The builders are the thoughts that construct a self out of judgment and achievement, and they throw away the one thing they cannot use, the innocence that asks nothing and proves nothing. Rejected, it becomes the corner where the true structure holds.
The reversal is love's doing, not the ego's, and it looks marvelous only because the mind had learned to see backward. Restore the discarded stone and the shaky edifice of separation is quietly replaced by something that cannot fall.
Builders: the ego's thoughts that construct a self from judgment and effort.
Stone: the mind's innocence, discarded as useless and made the ground of its peace.
Head of the corner: the place where the true self holds everything together once its worth is restored.