function y_new = euler_step ( x, y, f, dx )
%
%  function y_new = euler_step ( x, y, f, dx )
%
%  EULER_STEP uses Euler's method to estimate Y_NEW, the value of the
%  solution of an ODE at X+DX, given the solution Y at X, and
%  the name of a routine F(X,Y) which evaluates the derivative function.
%
  y_new = y + dx * feval ( f, x, y );
