Add FUN examples to x2y header

+tools/x2y.m

@@ -14,6 +14,14 @@
 %  quantity in the first dimension of the grid and B is the quantity in the
 %  second dimension. FUN must be a linear function in logspace for A and B. 
 %  
+%    FUN examples:
+%    A. mass-mobility > effective density-mobility
+%       FUN = @(a, b) 6 .* a ./ (pi .* b .^ 3) .* 1e9;
+%    B. mrBC-mp > frBC-mp
+%       FUN = @(a, b) a ./ b;
+%    C. mrBC-mp > fnrBC/mp
+%       FUN = @(a, b) 1 - a ./ b;
+%  
 %  Y = tools.x2y(X, GRID_X, FUN, DIM) applies the transformation while
 %  preserving the quantity in the DIM dimension of the input grid. For
 %  example, for mass-mobility distributions and the default of DIM = 2, the