[Here it is](https://github.com/ResilientScience/rofl/blob/1d8ae0ab30f5f44e1d4fc9cd8ff07812fcdd0af0/rofl/rofl_ast.py#L73). Notice the workarounds that are needed because `f()` doesn't work. It makes sense for Python functions, so it should make sense for SymPy functions.