P is the phase angle of pseudorotation for five-membered rings.
               For ribose and deoxyribose sugars in nucleic
               acids
                          (tau4 +tau1)-(tau3+tau0)
               P = ATAN (-------------------------)
                           2tau2 (sin 36+sin 72)
               If tau2 is <0, then P=P+180 degree (Altona & Sundaralingam,
               1972).
               Ref: Altona, C. & Sundaralingam, M. (1972).
               J. Am. Chem. Soc. 94, 8205-8212.