samer@44: % with_field - Apply operator to structure
samer@44: % with_field :: 
samer@44: %     N:string ~'field name',
samer@44: %     (A->B)   ~'function'
samer@44: % -> (struct { N : A } -> struct { N : B}).
samer@44: function g=with_field(nm,f)
samer@44: 	g=@op;
samer@44: 	function x=op(x), x.(nm)=f(x.(nm)); end
samer@44: end