Right, associativity is the simplest case because the structure is visible directly in one example.

The system needs multiple examples when there is more than one varying part and a single example is ambiguous. A simple example is wrapping a function call. With:

    doThing(x) → log(doThing(x))
    process(y) → log(process(y))

the system learns that: the function name varies the argument varies he outer log(…) is constant

From that it infers the general rule and applies it to new inputs. A single example would not be enough to disambiguate that pattern.