diff --git a/README.md b/README.md
index 76cfb40631c2aaebca96e28e296cea06cbdb7c9f..839123047f2f1c1bc5413a732735a18d83566bf9 100644
--- a/README.md
+++ b/README.md
@@ -68,10 +68,33 @@ the builtin `\ALL` set, the set of every function in the program.
 One can also use the `\in_file` function to select all functions defined in a
 given file.
 
+Finally, it is possible to use `\callees(s)` to refer to the functions in `s` and
+all their callees recursively. Of course, `\callers(s)` will include `s` and all
+their callers recursively up to the main entry point. In presence of function
+pointers, MetAcsl will rely on the Callgraph plugin, which either uses the results of Eva
+if an analysis has been launched before, or considers the set of all functions that
+have a type compatible with the pointer.
+
 For example, the target set `\diff(\ALL, \union({foo, bar},
 \in_file("main.c")))` describes all functions except `foo`, `bar` or any
 function defined in `main.c`.
 
+By default, if a function name does not correspond to a function in the
+program under analysis, Frama-C will be aborted. This can be changed by
+setting the status of warning category `unknown-func` to a suitable value,
+e.g. `-meta-warn-key unknown-func=enable`, to keep a warning when such a
+case occurs, or `-meta-warn-key unknown-func=disable` to silently ignore this
+fact.
+
+Note that if you disable the warning in order to use the same meta-properties on
+a full code base and on a reduced one where some functions have been removed, the
+resulting targets in the reduced case might not be the intersection of the targets
+in full case with the functions of the reduced case. For instance, if `f1` calls `f2`
+and only `f2` is included in the reduced case, `\callees({f1})` will be the empty set
+(since `f1` is ignored as unknown function) in the reduced case, and `{ f1, f2 }` in
+the full case. Similarly `\diff(\ALL,\callees({f1}))` will _include_ `f2` in the reduced case,
+but not in the full case.
+
 ### Context <a name="context"></a>
 
 The context is the situation in a target function in which the property must