[set varName ?newValue?]

  if null $newValue goto read_var

  .local pmc set
  set = get_root_global ['_tcl'], '__set'
  $R = set($varName, $newValue)
  goto end

read_var:
  .local pmc read
  read = get_root_global ['_tcl'], '__read'
  $R = read($varName)

end:
  $R = clone $R
