I've only found some indirect clue from eta expansion
SimpleExpr ::= SimpleExpr1
`_' The expression _ is well-formed if is of method type or if is a call-by-name parameter.
If is a method with parameters, _ represents converted to a function type by eta expansion.
If is a parameterless method or call-by-name parameter of type =>, _ represents the function of type () => , which evaluates when it is applied to the empty parameterlist ().
So i guess eta is short for expression to anonymous function? Am i right?