#define NRANSI#include "nrutil.h"void mmid(float y[], float dydx[], int nvar, float xs, float htot, int nstep,	float yout[], void (*derivs)(float, float[], float[])){	int n,i;	float x,swap,h2,h,*ym,*yn;	ym=vector(1,nvar);	yn=vector(1,nvar);	h=htot/nstep;	for (i=1;i<=nvar;i++) {		ym[i]=y[i];		yn[i]=y[i]+h*dydx[i];	}	x=xs+h;	(*derivs)(x,yn,yout);	h2=2.0*h;	for (n=2;n<=nstep;n++) {		for (i=1;i<=nvar;i++) {			swap=ym[i]+h2*yout[i];			ym[i]=yn[i];			yn[i]=swap;		}		x += h;		(*derivs)(x,yn,yout);	}	for (i=1;i<=nvar;i++)		yout[i]=0.5*(ym[i]+yn[i]+h*yout[i]);	free_vector(yn,1,nvar);	free_vector(ym,1,nvar);}#undef NRANSI