The reserved words in R's parser are
if else repeat while
  function for in
  next break
TRUE FALSE NULL
  Inf NaN
  NA NA_integer_ NA_real_
  NA_complex_ NA_character_
… and ..1, ..2 etc, which are used to refer to
  arguments passed down from a calling function, see ....
Reserved words outside quotes are always parsed to be
  references to the objects linked to in the ‘Description’, and
  hence they are not allowed as syntactic names (see
  make.names).  They are allowed as non-syntactic
  names, e.g.inside backtick quotes.