Build the canonical Tree form of an expression (structure: Heads/Subexpressions/Atoms/HeadTrees)
ExpressionTree[expr]
ExpressionTree[expr, struct]
ExpressionTree[f[a, b, c[p, q, r]]]
→ Tree[f, {Tree[a, None], Tree[b, None], Tree[c, {Tree[p, None], Tree[q, None], Tree[r, None]}]}]ExpressionTree[1, "Heads"]
→ Tree[1, None]ExpressionTree[1, "Atoms"]
→ Tree[1, None]