class Rufo::Formatter

def simple_string(node)

and return the simple string inside.
[:string_literal, [:string_content, [:@tstring_content, "abc", [...]]]]
For simple string formatting, look for nodes like: 
def simple_string(node)
  inner = node[1][1..-1]
  return if inner.length > 1
  inner = inner[0]
  return "" if !inner
  return if inner[0] != :@tstring_content
  string = inner[1]
  string
end