'Make all coefficients in ``f`` positive.
Examples
>>> from sympy import Poly
>>> from sympy.abc import x
>>> Poly(x**2 - 1, x).abs()
Poly(x**2 + 1, x, domain=\'ZZ\')'

def abs(f):

    if hasattr(f.rep, 'abs'):
        result = f.rep.abs()
    else:
        raise OperationNotSupported(f, 'abs')
    return f.per(result)
