User blog comment:Hyp cos/R function II/@comment-2033667-20150406074146/@comment-11227630-20150406075549

No. In my definition, 2R{{0+2}1+2} is not valid because the lbrace of the inner brace is immediately after the lbrace of the outer one; but it is valid according to your definition because 0 is a numeric term, and {0+2} is a bracketed term (that's still valid in my definition), 1 is a numberic term, so {{0+2}1+2} is a bracketed term.

Well, you say "R-expressions separated by commas", but is {0,{1+1},1+1} a valid R-expression ?(in my definition: no)