What's the meaning of this service specification What's the meaning of the example statement here?

"A service specification S describes an interface: it specifies the behavior of certain program elements (functions or subprograms) through statements expressed in program logic. For instance, these might be such statements:


[...]
x, y : N[{x>0} y := x * x {y > x}]


"

The quote is from the article  "On The Hourglass Model"
By Micah Beck,
Communications of the ACM, July 2019, Vol. 62 No. 7, Pages 48-57
That means:


For all natural numbers x and y,

if the value of variable x is greater than zero
then after the assignment statement "y := x*x" is executed
the predicate "y > x" will be true.