9999999999999999.0 – 9999999999999998.0

[PMunch]

This works with MAPM as well. As everyone else have already pointed out it's 
just a quirk of how IEEE floating point numbers work. MAPM has some nice 
convenience features as well, so this works and produces the number 1:
    
    
    import mapm
    
    var c = 9999999999999999.0'm - 9999999999999998.0'm
    echo c
    
    
    Run