function parseString (state) {
    let node, str

    if (state.token === '"' || state.token === "'") {
      str = parseStringToken(state, state.token)

      // create constant
      node = new ConstantNode(str)

      // parse index parameters
      node = parseAccessors(state, node)

      return node
    }

    return parseMatrix(state)
  }
Parse a single or double quoted string.
@return {Node} node
@private