'Returns l1 norm of ``f``.
Examples
>>> from sympy import Poly
>>> from sympy.abc import x
>>> Poly(-x**2 + 2*x - 3, x).l1_norm()
6'

def l1_norm(f):

    if hasattr(f.rep, 'l1_norm'):
        result = f.rep.l1_norm()
    else:
        raise OperationNotSupported(f, 'l1_norm')
    return f.rep.dom.to_sympy(result)
