set the values viewed through the traversal and returns its *new* values
set the values viewed through the traversal and ignores both values
set the values viewed through the traversal and returns its *old* values
extracts the values viewed through the traversal
extracts the values viewed through the traversal and applied f
over it
modify the values viewed through the traversal and returns its *new* values
modify the values viewed through the traversal and ignores both values
modify the values viewed through the traversal and returns its *old* values
alias for toState
transforms a PTraversal into a State