{ nodeType: 'Program',
  block:
   { nodeType: 'Block',
     statements:
      [ { nodeType: 'ConcatOp',
          left:
           { nodeType: 'ConcatOp',
             left: { nodeType: 'String', data: 'a' },
             right: { nodeType: 'Identifier', data: 'b' } },
          right: { nodeType: 'String', data: 'cdef' } } ] } }

-- basically, it grows to the left

        concat
       /      \
   concat   "cdef"
  /      \
"a"       b