update README

README.md

@@ -52,8 +52,8 @@ Extend Scryer Prolog to include the following, among other features:
       "Delimited Continuations for Prolog").
 - [x] Tabling library based on delimited continuations
       (documented in "Tabling as a Library with Delimited Control").      
-- [ ] A _redone_ representation of strings as difference list of
-      chars, using a packed internal representation (_in progress_).      
+- [x] A _redone_ representation of strings as difference list of
+      chars, using a packed internal representation.
 - [ ] clp(B) and clp(ℤ) as builtin libraries (_in progress_).
 - [ ] Streams and predicates for stream control (_in progress_).
 - [ ] An incremental compacting garbage collector satisfying the five
@@ -122,128 +122,6 @@ and BSD variants on which Rust runs. Windows support hinges on
 rustyline and Termion being functional in that environment, which to
 my knowledge is not presently the case.
 
-## Built-in predicates
-
-The following predicates are built-in to Scryer.
-
-* Arithmetic support:
-    * `is/2` works for `(+)/{1,2}`, `(-)/{1,2}`, `(*)/2`, `(//)/2`, `(**)/2`,
-      `(^)/2`, `(div)/2`, `(/)/2`, `(rdiv)/2`, `(xor)/2`, `(rem)/2`,
-      `(mod)/2`, `(/\)/2`, `(\/)/2`, `(>>)/2`,`(<<)/2`, `(\)/1`,
-      `abs/1`, `sin/1`, `cos/1`, `tan/1`, `asin/1`, `acos/1`,
-      `atan/1`, `atan2/2`, `log/1`, `exp/1`, `sqrt/1`, `float/1`,
-      `truncate/1`, `round/1`, `floor/1`, `ceiling/1`, `pi/0`,
-      `min/1`, `max/1`, `gcd/2`, `sign/1`
-    * Comparison operators: `>`, `<`, `=<`, `>=`, `=:=`, `=\=`.
-* `(:)/2`
-* `(@>)/2`
-* `(@>=)/2`
-* `(@=<)/2`
-* `(@<)/2`
-* `(\+)/1`
-* `(==)/2`
-* `(\==)/2`
-* `(=)/2`
-* `(\=)/2`
-* `(=..)/2`
-* `(->)/2`
-* `(;)/2`
-* `abolish/1`
-* `acyclic_term/2`
-* `append/3`
-* `arg/3`
-* `asserta/1`
-* `assertz/1`
-* `atom/1`
-* `atomic/1`
-* `atom_chars/2`
-* `atom_codes/2`
-* `atom_concat/3`
-* `atom_length/2`
-* `bagof/3`
-* `bb_b_put/2`
-* `bb_get/2`
-* `bb_put/2`
-* `between/3`
-* `call/1..62`
-* `call_cleanup/2`
-* `call_with_inference_limit/3`
-* `call_residue_vars/2`
-* `can_be/2`
-* `catch/3`
-* `clause/2`
-* `compare/3`
-* `compound/1`
-* `copy_term/{2,3}`
-* `current_predicate/1`
-* `current_op/3`
-* `cyclic_term/1`
-* `dif/2`
-* `expand_goal/2`
-* `expand_term/2`
-* `fail/0`
-* `false/0`
-* `findall/{3,4}`
-* `float/1`
-* `foldl/{4,5}`
-* `forall/2`
-* `freeze/2`
-* `functor/3`
-* `gen_int/1`
-* `gen_nat/1`
-* `get_char/1`
-* `goal_expansion/2`
-* `ground/1`
-* `halt/0`
-* `integer/1`
-* `is_list/1`
-* `is_partial_string/1`
-* `keysort/2`
-* `length/2`
-* `maplist/2..9`
-* `member/2`
-* `memberchk/2`
-* `must_be/2`
-* `nl/0`
-* `nonvar/1`
-* `number_chars/2`
-* `number_codes/2`
-* `numbervars/2`
-* `numlist/{2,3}`
-* `once/1`
-* `op/3`
-* `partial_string/2`
-* `phrase/{2,3}`
-* `rational/1`
-* `read/1`
-* `repeat/{0,1}`
-* `retract/1`
-* `reverse/2`
-* `same_length/2`
-* `select/3`
-* `setof/3`
-* `setup_call_cleanup/3`
-* `sort/2`
-* `string/1`
-* `sub_atom/5`
-* `subsumes_term/2`
-* `sumlist/2`
-* `term_expansion/2`
-* `term_variables/2`
-* `throw/1`
-* `true/0`
-* `unify_with_occurs_check/2`
-* `use_module/{1,2}`
-* `user:goal_expansion/2`
-* `user:term_expansion/2`
-* `var/1`
-* `variant/2`
-* `wam_instructions/2`
-* `write/1`
-* `write_canonical/1`
-* `writeq/1`
-* `write_term/2`
-
 ## Tutorial
 To enter a multi-clause predicate, the directive "[user]" is used.
 
@@ -321,22 +199,26 @@ New operators can be defined using the `op` declaration.
 
 ### Partial strings
 
-Scryer has two specialized, non-ISO builtin predicates for handling
-so-called "partial strings". Partial strings imitate difference lists
-of characters, but are much more space efficient. This efficiency
-comes at the cost of full generality -- you cannot unify the tail
-variables of two distinct partial strings, because their buffers will
-always be distinct.
+Scryer has three specialized non-ISO predicates for handling so-called
+"partial strings." Partial strings imitate difference lists of
+characters, but their characters are packed in UTF-8 format, a much
+more efficient alternative to how lists of characters are represented
+in many other Prologs.
+
+To use partial strings, the `non_iso` library must be loaded:
+
+`?- use_module(library(non_iso)).`
 
 If `X` is a free variable, the query
 
-`?- partial_string("abc", X), X = [a, b, c | Y], is_partial_string(X),
-is_partial_string(Y).`
+`?- partial_string("abc", X, _), X = [a, b, c | Y], partial_string(X),
+partial_string_tail(X, Tail), Tail == Y.`
 
-will succeed. Further, if `Y` a free variable, unifying `Y` against
-another string, "def" in this case, produces the equations
+will succeed, posting:
 
-`X = [a, b, c, d, e, f], Y = [d, e, f].`
+`Tail = Y, X = [a,b,c|Y].`
+
+By all appearances, partial strings are plain Prolog lists.
 
 ### Modules