modulo calculation: inverse of a number

[parisse]

The class of x in Z/nZ is represented by x %% n, you can do arithmetic operations on these objects (including polynomials having them as coefficients).
a:=(3 %% 14)^-1
The class residue is represented with x between -n/2 and n/2.
Use %% 0 to extract the integer (for example a %% 0 will return 5 here).