I don't know anything about TAP, but:

plan        ::= '1..' nonNegativeInteger "\n"
[download]

allows:

1..0
[download]

doesn't it? Is that intended, or is what you are after more like 1..n where n is > 1?

Update: The following two lines constrain plan as described above (with a little guessing at syntax):

pluralInteger      ::= (digit - ('0', '1')) | digit digit+

plan        ::= '1..' pluralInteger "\n"
[download]