separate linear indexing transformation from other indexing trans.
linear indexing transformation comes a lot in benchmarks, and it's
easy to transform to the normal array index, and rigorous indexing
transformations are not very readable, leave rigorous indexing
transformations to those cases in which there is no way to perform
direct index transformation.
e.g.
a = ones(3,3,3);
b = a(2,:); % cannot replace this statement with only one statement.
c = a(2,2); % while linear indexing can be replaced with one
            % corresponding statement, like c = a(2,2,1).