@classmethod
def get(cls, size=32, depth=1, clean=True):
    """Return a randomly generated expression
        @size: (optional) Operation size
        @depth: (optional) Expression depth
        @clean: (optional) Clean expression cache between two calls
        """
    if clean:
        cls.generated_elements = {}
    got = cls._gen(size=size, depth=depth)
    if clean:
        cls.generated_elements = {}
    return got
