you can reduce an individual function to a a directed graph (not a tree). But once you have more than one function? That's graphs referencing graphs. That defin

you can reduce an individual function to a a directed graph (not a tree). But once you have more than one function? That's graphs referencing graphs. That definitely doesn't reduce to a tree. The only parts of a program that reduce to a tree are incidental to their functionality - the nesting-block structure of procedural, C-derived languages, and the tree structure of a filesystem.