Parse "1 2 +" as an expr
 Try 'number'
  Parse 'number', left with "2 +"
  Parse 'end', PANIC!
 Try 'expr unary'
  Parse "1 2 +" as an expr
   Try 'number'
    Parse 'number', left with "2 +"
    Parse 'end', PANIC!
   Try 'expr unary'
   ...