If you create new operators, keep in mind that their precedence and associativity depend on the first character in the operator name (ignoring leading . and $).

That is:

• += has the same precedence as +
• *% has the same precedence as *
• .! has the same precedence as ! (this is infix)

Prefix operators start with !, ? and ~ (except those starting with += ).

> let (+++) x y = x * x + y * y;;
val ( +++ ) : int -> int -> int

> 4 +++ 3;;
val it : int = 25

> let (!+) x = abs x;;
val ( !+ ) : int -> int

> !+ -4;;
val it : int = 4

• Precedence and Operators (F# Draft Language Specification)