Method code for $math.runge()

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  1: arg x, y, h, f, data;
  2: var k1, k2, k3, k4, s;
  3: 
  4: // returns the next timestep for differential equation y'=f(x,y,data)
  5: s = sender();
  6: k1 = s.f(x, y, data);
  7: k2 = s.f(x + 0.5 * h, .add(y, .scale(0.5 * h, k1)), data);
  8: k3 = s.f(x + 0.5 * h, .add(y, .scale(0.5 * h, k2)), data);
  9: k4 = s.f(x + h, .add(y, .scale(h, k3)), data);
 10: return .add(y, .scale(h / 6.0, .add(.add(k1, .scale(2.0, .add(k2, k3))), k4)));

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// Miroslav Silovic
// Created 19-Oct-1996 as a part of ColdCore, see: @help Credit