code: def requiredOneInGroup(col_name, group, dm, df, *args):
      
      if col_name in df.columns:
          # if the column name is present, return nothing
          return None
      else:
          # if the column name is missing, return column name
          return col_name
docs: If col_name is present in df, the group validation is satisfied.
      If not, it still may be satisfied, but not by THIS col_name.
      If col_name is missing, return col_name, else return None.
      Later, we will validate to see if there is at least one None (non-missing)
      value for this group.