You cannot assign to this predefined symbol, or the predefined symbol is not accessible here. Example:
pixel = pixel + (2,0)
The solution is to introduce a new temporary variable and use it instead of the predefined symbol in the rest of the formula.
complex pix = pixel + (2,0)
Note: some old Fractint formulas write a lot to predefined symbols. You should rewrite these formulas if you want to compile them with Ultra Fractal.
See Also
Errors