s := "x" ----> "x"
s ==:= "x" ----> "x"
s ----> "x"
s := "x" ----> "x"
s ==:= "xx" ----> none
s ----> "x"
s := "x" ----> "x"
s ==:= "X" ----> none
s ----> "x"
s := "x" ----> "x"
s ==:= "abc" ----> none
s ----> "x"
s := "x" ----> "x"
s >>=:= "x" ----> "x"
s ----> "x"
s := "x" ----> "x"
s >>=:= "xx" ----> none
s ----> "x"
s := "x" ----> "x"
s >>=:= "X" ----> "X"
s ----> "X"
s := "x" ----> "x"
s >>=:= "abc" ----> "abc"
s ----> "abc"
s := "x" ----> "x"
s >>:= "x" ----> none
s ----> "x"
s := "x" ----> "x"
s >>:= "xx" ----> none
s ----> "x"
s := "x" ----> "x"
s >>:= "X" ----> "X"
s ----> "X"
s := "x" ----> "x"
s >>:= "abc" ----> "abc"
s ----> "abc"
s := "x" ----> "x"
s <<=:= "x" ----> "x"
s ----> "x"
s := "x" ----> "x"
s <<=:= "xx" ----> "xx"
s ----> "xx"
s := "x" ----> "x"
s <<=:= "X" ----> none
s ----> "x"
s := "x" ----> "x"
s <<=:= "abc" ----> none
s ----> "x"
s >>:= 0 ----> "0"