code: function startNewMode(mode, lexeme) {
      var node
  
      if (mode.className) {
        node = build(mode.className, [])
      }
  
      if (mode.returnBegin) {
        modeBuffer = ''
      } else if (mode.excludeBegin) {
        addText(lexeme, currentChildren)
  
        modeBuffer = ''
      } else {
        modeBuffer = lexeme
      }
  
      // Enter a new mode.
      if (node) {
        currentChildren.push(node)
        stack.push(currentChildren)
        currentChildren = node.children
      }
  
      top = Object.create(mode, {parent: {value: top}})
    }
docs: Start a new mode with a `lexeme` to process.