txr: function span variable must match existing value.

* match.c (h_var_compat): New function; verbatim copy
of existing h_var prior to this commit.
(h_var): If a variable has an existing binding, but
is a function spanning match, do not substitute it with text.
Handle it with the ordinary case, in which we now use
dest_bind instead of cons.
(v_var): Similarly, here, we must also use dest_bind, rather
than always freshly binding the variable.
(match_compat_fixup): For 272 compatibility, substitute
h_var_compat for h_var in the horizontal directive table.

* tests/010/span-var.txr: New test cases.

* txr.1: Documentation updated and also improved overall.
The behavior when a variable has an existing value is
clarified for the regex and fixed field case.
Also update and condense compat notes for 272.

1 files changed, 13 insertions, 0 deletions

diff --git a/tests/010/span-var.txr b/tests/010/span-var.txr
index 5f5faa6c..b7142b79 100644
--- a/tests/010/span-var.txr
+++ b/tests/010/span-var.txr
@@ -13,3 +13,16 @@
 @{w (fun2 x "a")}@y
 @(require (equal w "a"))
 @(require (equal y "b"))
+@(next :list '("a" "a" "a" "b" "c"))
+@(bind d ("d"))
+@(cases)
+@  {d (fun "x" "a")}
+@(or)
+@  (require "get here")
+@(end)
+@(next :string "ab")
+@(cases)
+@  {d (fun2 "x" "a")}
+@(or)
+@  (require "get here")
+@(end)