Merge pull request #2034 from xushiwei/q

mini spec: Notation

Author: xushiwei

doc/spec-mini.md

@@ -1,6 +1,35 @@
 Go+ Specification for STEM Education
 =====
 
+## Notation
+
+The syntax is specified using a [variant](https://en.wikipedia.org/wiki/Wirth_syntax_notation) of Extended Backus-Naur Form (EBNF):
+
+```go
+Syntax      = { Production } .
+Production  = production_name "=" [ Expression ] "." .
+Expression  = Term { "|" Term } .
+Term        = Factor { Factor } .
+Factor      = production_name | token [ "…" token ] | Group | Option | Repetition .
+Group       = "(" Expression ")" .
+Option      = "[" Expression "]" .
+Repetition  = "{" Expression "}" .
+```
+
+Productions are expressions constructed from terms and the following operators, in increasing precedence:
+
+```go
+|   alternation
+()  grouping
+[]  option (0 or 1 times)
+{}  repetition (0 to n times)
+```
+
+Lowercase production names are used to identify lexical (terminal) tokens. Non-terminals are in CamelCase. Lexical tokens are enclosed in double quotes `""` or back quotes ``.
+
+The form `a … b` represents the set of characters from a through b as alternatives. The horizontal ellipsis `…` is also used elsewhere in the spec to informally denote various enumerations or code snippets that are not further specified. The character … (as opposed to the three characters `...`) is not a token of the Go+ language.
+
+
 ## Comments
 
 Comments serve as program documentation. There are three forms: