An access x$f written where x was still open reads the field of whatever x is bound to once that binding is known, so binding the read at readId to a definition holding envState links the access to that field.
x$f
x
readId
envState
An access
x$fwritten wherexwas still open reads the field of whateverxis bound to once that binding is known, so binding the read atreadIdto a definition holdingenvStatelinks the access to that field.