y = a1 + a2 x1 + a3 x12 + a4 x2 + a5 x22 + a6 x1 x2 + a7 x1^3 + a8 x2^3 + a9 x12 x2 + a10 x1 x22
function bicubic input Real a[10] "Coefficients"; input Real x1 "Independent variable"; input Real x2 "Independent variable"; output Real y "Result"; end bicubic;