def _nt_sum(cobj, prop, theta):
    """
    Create sum expressions in n-t forms (sum(n[i]*theta**t[i]))

    Args:
        cobj: Component object that will contain the parameters
        prop: name of property parameters are associated with
        theta: expression or variable to use for theta in expression

    Returns:
        Pyomo expression of sum term
    """
    # Build sum term
    i = 1
    s = 0
    while True:
        try:
            ni = getattr(cobj, f"{prop}_coeff_n{i}")
            ti = getattr(cobj, f"{prop}_coeff_t{i}")
            s += ni * theta**ti
            i += 1
        except AttributeError:
            break
    return s