def maximum_of(*column_names, **extra_columns):
    """
    Return the maximum value over the supplied columns e.g

      max_value=patients.maximum_of("some_column", "another_column")

    Additional columns can be defined within the function call which will be
    used in computing the maximum but won't themselves appear in the output:

      max_value=patients.maximum_of(
          "some_column",
          another_colum=patients.with_these_medications(...)
      )
    """
    aggregate_function = "MAX"
    column_names = column_names + tuple(extra_columns.keys())
    return "aggregate_of", locals()