Add language documentation in commandline help

bin/core/imag-ids/src/ui.rs

@@ -46,5 +46,6 @@ pub fn build_ui<'a>(app: App<'a, 'a>) -> App<'a, 'a> {
                          .value_names(&["QUERY"])
                          .help("Query the header of the entries and filter them"))
                    )
+        .after_help(include_str!("../static/language-doc.md"))
 }
 

bin/core/imag-ids/static/language-doc.md

@@ -0,0 +1,51 @@
+Language documentation for the imag-ids query language
+======================================================
+
+The query language imag-ids supports is rather simple.
+It can be used to filter the printed imag ids by the values in the header of the
+entries. It has no way to access the content of the entries (yet).
+
+Following is a BNF-like structure shown how the language definition works.
+This definition may change over time, as the language grews more powerful.
+
+```ignore
+query = filter (operator filter)*
+
+filter = unary? ( (function "(" selector ")" ) | selector ) op val
+
+unary = "not"
+
+op =
+    "is"  |
+    "in"  |
+    "=="  |
+    "eq"  |
+    "!="  |
+    "neq" |
+    ">="  |
+    "<="  |
+    "<"   |
+    ">"   |
+    "any" |
+    "all"
+
+val = val | listofval
+
+val         = string | int | bool
+listofval   = "[" (val ",")* "]"
+
+operator =
+    "or"      |
+    "or_not"  |
+    "and"     |
+    "and_not" |
+    "xor"
+
+function =
+    "length" |
+    "keys"   |
+    "values"
+```
+
+A "string" quoted with double-quotes.
+A "val" does not yet support floats.