Index: trunk/doc/design/04_attrib.html
===================================================================
--- trunk/doc/design/04_attrib.html	(revision 3940)
+++ trunk/doc/design/04_attrib.html	(revision 3941)
@@ -28,7 +28,10 @@
 root omitted. Cschem itself and most plugins will ignore such pseudo-hierarchy,
 and will handle slash without side effects. Nevertheless GUI implementations
 may present attributes as a tree and attribute conventions will use the
-pseudo-hierarchy to group attributes.
+pseudo-hierarchy to group attributes. If the first character of the
+key is with - or +, that <a href="#des4:51">alters attribute merging
+during compilation.</a>. This is called attribute prefixing (the prefix
+is part of the key, e.g. when string comparing the key).
 
 <h2> 4.2. Attribute values </h2>
 
@@ -156,6 +159,17 @@
 
 <h2> 4.5. Attribute compilation process </h2>
 
+<p id="des4:51">{des4:51}
+First, all attributes of all concrete objects are copied into attributes
+of the abstract objects they contribute to. There are three ways the copy
+may happen depending on the first character of the concrete object attribute
+key:
+<ul>
+	<li> starting with minus (-) sign: attribute is <b>not</b> copied to the abstract object
+	<li> starting with a plus (+) sign: if attribute value is an array, values are appended on merge if there's an attribute collision (the plus prefix is removed from the key in the abstract object attribute)
+	<li> in any other case: user-user attribute collision is an error
+</ul>
+
 <h3> 4.5.1. Plugin invocation </h3>
 
 <p>{des4:20}
@@ -186,7 +200,10 @@
 There are only two ways attribute collision may happen:
 <ul>
 	<li> user-user collision: the same attribute of an abstract object is
-	     derived from two or more concrete objects
+	     derived from two or more concrete objects at the same priority
+	     (exception: with "-" prefix when the attribute is not copied/merged
+	     at all or array attribute keyed with the "+" prefix, when the list
+	     is appended)
 	<li> plugin-plugin collision: the same attribute of an abstract object is
 	     written twice by the same plugin
 </ul>
Index: trunk/doc/design/04_attrib_imp.html
===================================================================
--- trunk/doc/design/04_attrib_imp.html	(revision 3940)
+++ trunk/doc/design/04_attrib_imp.html	(revision 3941)
@@ -225,3 +225,18 @@
 the symlink dereference, thus will end up as dip8.
 
 
+<h3> 4.4.3. example on attribute key prefixes </h3>
+<p>{imp4:26}
+In slotting, multiple (concrete) symbols have the same name and will be merged
+into the same (abstract) component. Most attributes, like footprint, are
+either specified only once, or if they are specified in multiple symbols
+they have same value so there's no collision. 
+
+<p>{imp4:27}
+However, when using devmap, the user also needs to specify the slot
+number, per symbol. Normally this would be slot=1, slot=2, slot=3, ...
+attributes, but these would collide during merge. The solution is to
+prefix keys with a minus sign, so symbol attributes are -slot=1, -slot=2,
+-slot=3. Minus prefixed conrete attributes are ignored during compilation,
+the resulting component does not have a slot attribute and there's no collision.
+