  In this paper, we present an accurate and numerically efficient method to
implement the GN and EGN nonlinearity prediction methods when the power
evolution along the fiber is in an arbitrary form. This approach will provide
us with a reliable tool to efficiently use GN and Enhanced GN (EGN) methods for
analysis of the nonlinearity for ultrawideband coherent fiber optic WDM systems
(C+L band or even wider bandwidth systems) in the presence of the Inter-channel
Stimulated Raman Scattering (ISRS) and systems using forward-pumped or/and
backward-pumped Raman amplification.
