set the value viewed through the lens and returns its *new* value
set the value viewed through the lens and ignores both values
set the value viewed through the lens and returns its *old* value
extracts the value viewed through the lens
extracts the value viewed through the lens and applies f
over it
modify the value viewed through the lens and returns its *new* value
modify the value viewed through the lens and ignores both values
modify the value viewed through the lens and returns its *old* value
alias for toState
transforms a PLens into a State