s := ""                 # null string
s := string('A'--'A')   # ... converted from cset difference
s := char(0)[0:0]       # ... by slicing

s1 == ""                # lexical comparison, could convert s1 to string
s1 === ""               # comparison won't force conversion
*s1 = 0                 # zero length, however, *x is polymorphic
*string(s1) = 0         # zero length string

s1 ~== ""               # non null strings comparisons
s1 ~=== ""
*string(s1) ~= 0

s := &null              # NOT a null string, null type
/s                      # test for null type
\s                      # test for non-null type