Indeed, my program must act on itself by entering certain areas of code (in the form of a string) which it must itself execute.
Take the ring K of polynomials with four unknow x_00, x_10, x_01 and x_11
K. <x_00, x_10,="" x_01,="" x_11=""> = QQ [ ] g = let's say an element of K for i in range (1): for j in range (1): 2+ g.monomial_coefficient (x_ij)
Indeed the last command wants to add to 2 the coefficient of the monome x_ij. This is where his block.