(* Mathematica 7 *)
(* Clear memory *)
Clear["Global`*"];
(* Define a list of equations. *)
eqns = {x == 2y, x'[t] == 2t, x''[t] == 4t +1, y'[s] == 2z, y == t, y[m] == 2m, g[t] == x'[t]}
(* To find all equations with any derivative. *)
anyD = Position[ eqns, Derivative] (* Find positions. *);
pos = Take[#, 1][[1]]& /@ anyD (* Isolate first "address" element. *);
disp = Take[ eqns, {#}]& /@ pos (* Print out all eqns with any derivative. *)
(* To find equations with a specified derivative. *)
specificD = Position[ eqns,#]& /@ { _'''} (* Find positions. *);
pos = Take[#, 1][[1]]& /@ specificD[[1]] (* Isolate the "address" element. *);
disp = Take[ eqns, {#}]& /@ pos (* Print out the equations. *)
