The response is a point in the domain that is HTTP. A GET request is an identity relation on that point. A function from the domain of the request into the domain of the response exists, whether explicit or implied. The function is implied when a resource is created but not returned in the response.
Therefore the expression
identifies a point in the domain of HTTP. And likewise the expressions
are operations on the same point.
Compare this to arithmetic. The identity expression is
1 = 1
and operational expressions include
1 + 1
1 - 1
The equivalent algebraic identity expression would read as
"GET /index.html" = "response entity body data"
When we can see this algebra, then we have new eyes.