Draws horseshoes - left child, up over, down to right child

USAGE:

arbor.branch(x, y, node, branch) 

ARGUMENTS:

x
x coordinates of nodes
y
y coordinates of nodes
node
description of node number
branch
controls shape of horseshoe (1 is default, 0 has no shoulder)

VALUE:

Internal function, called by plot.arbor. Not usually called by user.