> mean( 1, 2, 3, 4, 5 );
                                   3

> mean( a + b, b + c, c + d, d + e, e + a );
                      2 a   2 b   2 c   2 d   2 e
                      --- + --- + --- + --- + ---
                       5     5     5     5     5

> mean(); # again, an exception is raised
Error, (in mean) numeric exception: division by zero