(vector z) = (vector x) + (scalar f) * (vector y)

 
/* sphau - augment one vector by another     */
/* vector z = vector x + scalar f * vector y */

void sphau(z,x,f,y) double z[3], x[3], f, y[3]; {
z[0]=x[0]+f*y[0];
z[1]=x[1]+f*y[1];
z[2]=x[2]+f*y[2];
}