P is the phase angle of pseudorotation for five membered rings.
This formulation is used for ribo and deoxyribo sugars in nucleic
acids.
(tau4 +tau1)-(tau3+tau0)
P = ATAN (-------------------------)
2tau2 (sin 36+sin 72)
If tau2 is <0, then P=P+180
This formulation is by Altona and Sundaralingam (1972), J.A.C.S.,
94, 8205-8212.