def clean(self, *, atol: float = 1e-9) -> Self:
        """Remove terms with coefficients of absolute value atol or less."""
        negligible = [
            v
            for v, c in self._terms.items()
            if not isinstance(c, sympy.Basic) and abs(complex(c)) <= atol
        ]
        for v in negligible:
            del self._terms[v]
        return self
Remove terms with coefficients of absolute value atol or less.
cirq-core/cirq/value/linear_dict.py