source 2007 Hisil--Carter--Dawson, plus common-subexpression elimination
parameter b
assume b = a - 1
parameter b2
assume b2 = 2 b
parameter b3
assume b3 = 3 b
parameter bb2
assume bb2 = 2 b b
compute E = S1^2
compute F = C1^2
compute G = E^2
compute H = F^2
compute J = G^2
compute K = H^2
compute L = (E+F)^2-H-G
compute M = L^2
compute N = (G+L)^2-J-M
compute P = (H+L)^2-K-M
compute R = bb2 J
compute Q = b2 N
compute T = b3 M
compute U = 2 P
compute V = 2 K
compute W = a U
compute Y = a Q
compute RV = R-V
compute RQ = R-Q
compute UV = U+V
compute TW = T+W
compute TY = T-Y
compute RQUV = RQ+UV
compute S3 = S1(RV+TW-2 UV)
compute C3 = C1(RV-TY+2 RQ)
compute D3 = D1(TW-RQUV)
compute Z3 = Z1(TY-RQUV)