let 1 : 1
let \. : 1
let \0 : 1
let \0. : 1
let \1.. : 1
let \1 : 1 # this one is good
let \1. : 1 # this one is good
let \9 : 1 # this one is good
let \a : 1
let \f() : 1
let a : 1