function resu = subsref(df, S)
 This function returns a subpart of a dataframe. It is invoked when
 calling df.field, df(value), or df{value}. In case of fields,
 returns either the content of the container with the same name,
 either the column with the same name, priority being given to the
 container. In case of range, selection may occur on name or order
 (not rowidx for rows). If the result is homogenous, it is
 downclassed. In case an extra field is given, is it used to
 determine the class of the return value. F.i., 
 df(1, 2, 'dataframe') 
 does not return a scalar but a dataframe, keeping all the meta-information

Package: dataframe