Represents a string object with interpolation inside backticks, e.g.:

  `lit#{next}`

The node is evaluated by duplicating the string stored in the ‘lit’ element, then iterating over the nodes stored in the ‘next’ element. Each node found should evalate to an string, and each resulting string is appended onto the original string.

The resulting string is executed in a subshell and the output from its stdout stored in a string, which becomes the result of the expression. Interpolation is represented with the EVSTR node.