  This work is motivated by a specific point of view: at short distances and
high energies the undoped and underdoped cuprates resemble the $\pi$-flux phase
of the t-J model. The purpose of this paper is to present a mechanism by which
pairing grows out of the doped $\pi$-flux phase. According to this mechanism
pairing symmetry is determined by a parameter controlling the quantum tunneling
of gauge flux quanta. For zero tunneling the symmetry is $d_{x^2-y^2}+id_{xy}$,
while for large tunneling it is $d_{x^2-y^2}$. A zero-temperature critical
point separates these two limits.
