| #### D.15.6.9 central2nd Procedure from library `finitediff.lib` (see finitediff_lib). Usage: central2nd(U1,U2,var); U1, U2 are the names of occuring derivatives, var is a variable in the basering; Return: type vector; gives a predefined approximation of the second-order-central-approximation as often used in literature; Note: see also forward,laxfrT,setinitials,scheme; Example:
    LIB "finitediff.lib";
    list D="Uxx","Ux","Utt","Ut","U";
    list P="lambda";
    list V="t","x";
    setinitials(V,D,P);
    central2nd(Uxx,U,x);
    ==> [(dx^2)*x,0,0,0,-x^2+2*x-1]
    central2nd(Utt,U,t);
    ==> [0,0,(dt^2)*t,0,-t^2+2*t-1]

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