Thank you for the clarification. When for example I use an expression like " pow(u,2) +6 " for example it works. But when i use a complicated expression ( atan, sqrt ... ) I get this error.
snippet of code :
"
md.add_macro('f(u)',"atan(u)")
md.add_nonlinear_term(mim,'f(u).Test_u', region)
"
