Function sharpen::tree_rollup::rollup_tree

pub fn rollup_tree<T, I, M>(
    input: I, 
    mapping: M
) -> Option<impl Iterator<Item = T>> where
    T: Node,
    I: IntoIterator<Item = T>,
    M: IntoIterator<Item = (usize, usize)>,
    M::IntoIter: DoubleEndedIterator, 

Roll up a tree given as a flat structure and a mapping {from child to parent} into a hierarchical structure.

Invariants for the arguments:

• Any parent must come before it's children inside input.
• The largest index in mapping should be inside of the bounds of input.
• mapping keys (child) must be greater than the associated value (parent)
• mapping keys should be sorted

Return value:

• None: Detected duplicated usage of id's (probably the mapping was invalid).
• Some(iter): The rolled-up tree, with only top-level children left at top-level.