set the value viewed through the Optional and returns its *new* value
set the value viewed through the Optional and ignores both values
set the value viewed through the Optional and return its *old* value, if there was one
extracts the value viewed through the optional
extracts the value viewed through the optional and applies f
over it
modify the value viewed through the Optional and return its *new* value, if there is one
modify the value viewed through the Optional and ignores both values
modify the value viewed through the Optional and return its *old* value, if there was one
alias for toState
transforms a POptional into a State