documentation update

diff --git a/NEWS b/NEWS
index a2ef2d1fac86302e9da0584923c1b8cb7f9f86de..2d6b662200e5131857b7a8be8e9af0e977d9d204 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -12,8 +12,13 @@ This file records noteworthy changes.
      gamma~0 in p.mu*gamma~mu should be p.0 but is returned as 0
    - using coeff(), tcoeff(), lcoeff() or collect() on elements of
      noncommutative products might return wrong or surprising results
-* Likewise, subs() can be used to substitute functions, tensors and indexed
-  expressions.
+* subs() no longer only substitutes symbols and indices but performs a more
+  general "syntactic substitution", i.e. it substitutes whole objects in sub-
+  expressions. You can subs((a+b)^2,a+b==3) and get 9, but subs(a+b+c,a+b==3)
+  doesn't do anything.
+  Limitations:
+   - substituting numerics (subs(expr, 2==4)) will not replace then in all
+     occurences; in general, you shouldn't substitute numerics, though
 * Added preliminary (re)implementations of color and clifford classes.
 * simplify_indexed(): contraction of symmetric and antisymmetric tensors
   is zero.