ref in f#
let a = ref 5 // allocates a new record on the heap
let b = a // b references the same record
b := 10 // modifies the value of 'a' as well!
let mutable a = 5 // mutable value on the stack
let mutable b = a // new mutable value initialized to current value of 'a'
b <- 10 // modifies the value of 'b' only!