Synopsis
		sfduffing1 < in.rsf > out.rsf restor=restor.rsf gamma=0.75 omega=1 kxi=1 x0=0 y0=0 pow1=1 pow2=3 phi=0. verb=n cosine=y

Duffing equation: x''/(omega^2)+0.5 x'/omega-x+x^3=gamma cos(omega t+phi)+kxi R(t)

Parameters
		bool cosine=y [y/n] if n need extenal input for periodic restoring force   float gamma=0.75 strength of external force   float kxi=1 adjustment for input signal   float omega=1 angular frequence of external force   float phi=0. phase of cosine signal unit=pi   int pow1=1 power of first non-linear restitution term   int pow2=3 power of second non-linear restitution term   string restor= auxiliary input file name   bool verb=n [y/n] verbosity flag   float x0=0 initial value of x0   float y0=0 initial value of y0