Many thanks to all who responded.
The solution was to remember that the argument for rtrim (as well as ups,
etc) is a string. One of the ways of specifying a string is to NAME a string
variable. What I was trying to do was to dereference the variable and pass
the value as a CONSTANT.
Silly me, this is the sort of thing that we forget somewhere around CompSci 101!
Oh well, the person who never forgets anything had nothing worth remembering
at any rate!
Tony