<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def hexagonal(a: float, c: float):
 """ Convenience constructor for a hexagonal lattice. Args: a (float):
 *a* lattice parameter of the hexagonal cell. c (float):
 *c* lattice parameter of the hexagonal cell. Returns: Hexagonal lattice of dimensions a x a x c. """

        return Lattice.from_parameters(a, a, c, 90, 90, 120)