`lookup(M,A,B)` -- provides the binary method named `M` for `(A,B)`. The first place to look is `Y#(M,A,B)` where `Y` is the younger of `A` and `B`. The search proceeds next with the parent of `B`, and so on.

`lookup(M,A,B,C)` -- provides the ternary method named `M` for `(A,B,C)`. The first place to look is `Y#(M,A,B,C)` where `Y` is the youngest of `A`, `B`, and `C`. The search proceeds with the parent of `C`, and so on.

`lookup(M,A,B,C,D)` -- provides the quaternary method named `M` for `(A,B,C,D)`. The first place to look is `Y#(M,A,B,C,D)` where `Y` is the youngest of `A`, `B`, `C`, and `D`. The search proceeds with the parent of `D`, and so on.

`lookup x` -- where `x` is a symbol or function, returns `x`.

If no method is found, then null is returned.

- # -- length or access to elements
- installMethod -- install methods
- youngest -- the youngest member of a sequence