    measure_column [ { { condition
                       | expr
                       | single_column_for_loop
                       }
                         [, { condition
                            | expr
                            | single_column_for_loop
                            }
                         ]...
                     | multi_column_for_loop
                     }
                   ]

    Note: The outer square brackets are part of the syntax.
          In this case, they do not indicate optionality.