Indent4Child appends more blank to the `indent` string.

PrettyIdentifier returns a pretty identifier which contains indent and tree node hierarchy indicator.

TreeBody indicates the current operator sub-tree is not finished, still has child operators to be attached on.

TreeGap is used to represent the gap between the branches of the tree.

TreeLastNode indicates this operator is the last child of the current sub-tree rooted by its parent.

TreeMiddleNode indicates this operator is not the last child of the current sub-tree rooted by its parent.

TreeNodeIdentifier is used to replace the treeGap once we need to attach a node to a sub-tree.