Except in the UK, where there often is a Mezzanine floor somewhere above the ground floor (usually, but not always, between the ground floor and the first floor).
Is there an Esolang that numbers its arrays with
0,M,1,2,3...?
A mezzanine is, by definition, a floor offset a non-integral number of storeys from the floors around it; a floor existing at a fractional floor number, in other words. A mezzanine "between the first and second floor" (in american parlance) would have a floor offset of 0.5 (or possibly ranging from 0.3 to 0.7, since mezzanines usually involve complex arrangements of stairs and landings.)
