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Jan 16th, 2018
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  1. rule = Times[f[x, t],
  2. Derivative[1, 0][y_][x, t]] :> (P[y] - D[y[x, t], t]);
  3.  
  4. A = (D[w[x, t], t]+f[x, t]*D[w[x, t], x])*(1 + (D[q[x, t], t] +f[x, t]*D[q[x, t], x]));
  5.  
  6. A /. rule
  7.  
  8. (* (1 + P[q]) P[w] *)
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