Index: env.scm
==================================================================
--- env.scm
+++ env.scm
@@ -41,38 +41,79 @@
 	      (val (cdr varval)))
 	  (if incremental (exec (sql db "DELETE FROM envvars WHERE context=? AND var=?") context var))
 	  (exec (sql db "INSERT INTO envvars (context,var,val) VALUES (?,?,?)") context var val)))
 	(get-environment-variables)))))
 
-;; apply contexts to current environment
+;; merge contexts in the order given
 ;;  - each context is applied in the given order
 ;;  - variables in the paths list are split on the separator and the components
 ;;    merged using simple delta addition
-;;
-(define (env:apply-contexts db basecontext contexts paths outputf formats)
-  
-  (for-each
-   (lambda (context)
-     (query
-      (for-each-row
-       (lambda (row)
-	 (let ((var  (car row))
-	       (vala (cadr row))
-	       (valb (caddr row)))
-	    ;;(print "var: " var " vala: " vala " valb" valb " paths: " paths)
-	   (if (assoc var paths) ;; this var is a PATH
-	       (let ((current (get-environment-variable var))) ;; use this NOT vala
-	         ;;(pp paths)
-                 ;;(pp var)
-		 (env:process-path-envvar var (cadr (assoc var paths)) current valb))
-	       (begin
-		 (setenv var valb))))))
-      (sql db "SELECT b.var,a.val,b.val FROM envvars AS a JOIN envvars AS b ON a.var=b.var WHERE a.context=? AND b.context=? AND a.val != b.val")
-      ;;(sql db "SELECT b.var,a.val,b.val FROM envvars AS a JOIN envvars AS b ON a.var=b.var WHERE a.context=? AND b.context=?")
-      basecontext context))
-   contexts))
-
+;;    returns a hash of the merged vars
+;;
+(define (env:merge-contexts db basecontext contexts paths)
+  (let ((result (make-hash-table)))
+    (for-each
+     (lambda (context)
+       (query
+	(for-each-row
+	 (lambda (row)
+	   (let ((var  (car row))
+		 (val  (cadr row)))
+	     (hash-table-set! result var 
+			      (if (and (hash-table-ref/default results var #f)
+				       (assoc var paths)) ;; this var is a path and there is a previous path
+				  (let ((sep (cadr (assoc var paths))))
+				    (env:merge-path-envvar sep (hash-table-ref results var) valb))
+				  valb)))))
+	(sql db "SELECT var,val FROM envvars WHERE context=?")
+	context))
+     contexts)
+    result))
+
+;;  get list of removed variables between two contexts
+;;
+(define (env:get-removed db contexta contextb)
+  (let ((result (make-hash-table)))
+    (query
+     (for-each-row
+      (lambda (row)
+	(let ((var  (car row))
+	      (val  (cadr row)))
+	  (hash-table-set! result var valb))))
+     (sql db "SELECT var,val FROM envvars WHERE context=? AND var NOT IN (SELECT var FROM envvars WHERE context=?)")
+     contexta contextb)
+    result))
+
+;;  get list of variables added to contextb from contexta
+;;
+(define (env:get-added db contexta contextb)
+  (let ((result (make-hash-table)))
+    (query
+     (for-each-row
+      (lambda (row)
+	(let ((var  (car row))
+	      (val  (cadr row)))
+	  (hash-table-set! result var valb))))
+     (sql db "SELECT var,val FROM envvars WHERE context=? AND var NOT IN (SELECT var FROM envvars WHERE context=?)")
+     contextb contexta)
+    result))
+
+;;  get list of variables in both contexta and contexb that have been changed
+;;
+(define (env:get-changed db contexta contextb)
+  (let ((result (make-hash-table)))
+    (query
+     (for-each-row
+      (lambda (row)
+	(let ((var  (car row))
+	      (val  (cadr row)))
+	  (hash-table-set! result var val))))
+     (sql db "SELECT var,val FROM envvars WHERE context=? AND val != (SELECT val FROM envvars WHERE var=? AND context=?)")
+     contexta contextb))
+  result)
+
+;;
 (define (env:blind-merge l1 l2)
   (if (null? l1) l2
       (if (null? l2) l1
 	  (cons (car l1) (cons (car l2) (env:blind-merge (cdr l1) (cdr l2)))))))
 
@@ -88,13 +129,10 @@
 ;;     (print "AFTER:    " (string-intersperse pathb-parts  "\n       "))
 ;;     (print "COMMON:   " (string-intersperse common-parts "\n       "))
     (string-intersperse final separator)))
 
 (define (env:process-path-envvar varname separator patha pathb)
-  (begin
-    (print "Process-path-envvar: " varname)
-  ) 
   (let ((newpath (env:merge-path-envvar separator patha pathb)))
     (setenv varname newpath)))
 
 (define (env:have-context db context)
   (> (query fetch-value (sql db "SELECT count(id) FROM envvars WHERE context=?") context)

Index: megatest.scm
==================================================================
--- megatest.scm
+++ megatest.scm
@@ -1896,10 +1896,24 @@
 	    (env:close-database db)
 	    (set! *didsomething* #t))
 	  (begin
 	    (debug:print 0 "ERROR: Parameter to -envcap should be <filename>=<context>. E.G. envdat=original, got: " envcap)
 	    (set! *didsomething* #t)))))
+
+;; delta "language" will eventually be res=a+b-c but for now it is just res=a-b 
+;;
+(let ((envdelta (args:get-arg "-envdelta")))
+  (if envdelta
+      (let ((match (string-match "([a-z]+)=([a-z\-,]+)" envdelta)))
+	(if match
+	    (let* ((resctx    (cadr match))
+		   (equn      (caddr match))
+		   (parts     (string-split equn "-"))
+		   (minuend   (car parts))
+		   (subtraend (cadr parts))
+		   (
+	    
 
 ;;======================================================================
 ;; Exit and clean up
 ;;======================================================================