samer@0: % obs_with - arrow to observe a signal while passing it through
samer@0: %
samer@0: % obs_with ::
samer@0: %    arrow({A},{},S) ~'arrow sink to observe values of type A',
samer@0: % -> arrow({A},{A},pair(empty,S)).
samer@0: 
samer@0: function b=obs_with(a), b=dup*first(a); end