class Sass::Engine

def build_tree(index)
  line, tabs = @lines[index]
  index += 1
  @line = index
  node = parse_line(line)
  has_children = has_children?(index, tabs)
  # Node is a symbol if it's non-outputting, like a constant assignment
  unless node.is_a? Tree::Node
    if has_children
      if node == :constant
        raise SyntaxError.new("Illegal nesting: Nothing may be nested beneath constants.", @line + 1)
      elsif node.is_a? Array
        # arrays can either be full of import statements
        # or attributes from mixin includes
        # in either case they shouldn't have children.
        # Need to peek into the array in order to give meaningful errors
        directive_type = (node.first.is_a?(Tree::DirectiveNode) ? "import" : "mixin")
        raise SyntaxError.new("Illegal nesting: Nothing may be nested beneath #{directive_type} directives.", @line + 1)
      end
    end
    index = @line if node == :mixin
    return node, index
  end
  node.line = @line
  if node.is_a? Tree::CommentNode
    while has_children
      line, index = raw_next_line(index)
      node << line
      has_children = has_children?(index, tabs)
    end
    return node, index
  end
  # Resolve multiline rules
  if node.is_a?(Tree::RuleNode)
    if node.continued?
      child, index = build_tree(index) if @lines[old_index = index]
      if @lines[old_index].nil? || has_children?(old_index, tabs) || !child.is_a?(Tree::RuleNode)
        raise SyntaxError.new("Rules can't end in commas.", @line)
      end
      node.add_rules child
    end
    node.children = child.children if child
  end
  while has_children
    child, index = build_tree(index)
    validate_and_append_child(node, child)
    has_children = has_children?(index, tabs)
  end
  return node, index
end