Why does the symbol $dt$ disappear in C/C++ code?

[jiwei08]

I wanna use the symbol dt given by model.grid.stepping_dim.spacing in the index of the matrix. But the symbol dt disappears in the C/C++ code when I check it using print(op). So what should I do to fix it? I am also confused with model.grid.stepping_dim.spacing and model.grid.time_dim.spacing. It seems like they both represent dt.

Any suggestions are appreciated!