LFUN
  protected mixed _add_id (mixed *prop, mixed str)

DESCRIPTION
  Add one or more strings to an array.

  Arguments:
    prop: the array to add the strings to (may be 0).
    str : the string or string array to add.

  Result:
    The given <prop> array, expanded by <str>.

  The function maps all strings to lowercase, removes excess spaces
  and makes sure that each string appears just once in the result
  array.

  This is an auxiliary function used in maintaining P_IDS, P_ADS,
  P_CLASS_IDS and P_CLASS_ADS and should not be called by hand.

IMPLEMENTED
  /std/thing/description