no completion when position lies outside of tokens
no completion when position lies outside of tokens

when the caret position lies outside of the parsed BEL tokens then
completions are not possible, hence return empty array.

this rule should still apply when completing through to the full BEL
statement, as in that case we will have more tokens in the list.

abargnesi/bel.rb,abargnesi/bel.rb,ncatlett/bel.rb,OpenBEL/bel.rb,OpenBEL/bel.rb,ncatlett/bel.rb,OpenBEL/bel.rb,abargnesi/bel.rb,nbargnesi/bel.rb,OpenBEL/bel.rb,abargnesi/bel.rb,nbargnesi/bel.rb,abargnesi/bel.rb,OpenBEL/bel.rb,OpenBEL/bel.rb,nbargnesi/bel.rb,nbargnesi/bel.rb,abargnesi/bel.rb,OpenBEL/bel.rb,nbargnesi/bel.rb,nbargnesi/bel.rb