Arguments
a
vector with current ancestor value (x & y).
d1
vector with current first daughter value (x & y).
d2
vector with current second daughter value (x & y).
s
time separating node being proposed from ancestor.
t1
time separating node being proposed from first daughter.
t2
time separating node being proposed from second ancestor.
sigma2x
current value for sigma2x.
sigma2y
current value for sigma2y.