The Interp function is a placeholder for polynomial interpolation. It is used in conjunction with mod and modp1.

The call Interp(x, y, v) mod p computes the polynomial of degree at most  in the name v that interpolates the points (x[1],y[1]), (x[2], y[2]),..., (x[n], y[n]) mod p. The points must be from a finite field.

The call modp1(Interp(x, y), p) computes the interpolation polynomial in the modp1 representation where x and y must be lists.

All the independent values in x must be distinct.  In other words, a particular value modulo p must not occur more than once in x.