Clean up grammar (#268)

GRAMMAR.md

@@ -11,22 +11,29 @@ tokens
 
 ```
 BACKTICK   = `[^`\n\r]*`
-COLON               = :
 COMMENT    = #([^!].*)?$
-NEWLINE             = \n|\r\n
-EQUALS              = =
-INTERPOLATION_START = {{
-INTERPOLATION_END   = }}
+DEDENT     = emitted when indentation decreases
+EOF        = emitted at the end of the file
+INDENT     = emitted when indentation increases
+LINE       = emitted before a recipe line
 NAME       = [a-zA-Z_][a-zA-Z0-9_-]*
-PLUS                = +
+NEWLINE    = \n|\r\n
 RAW_STRING = '[^'\r\n]*'
 STRING     = "[^"]*" # also processes \n \r \t \" \\ escapes
-INDENT              = emitted when indentation increases
-DEDENT              = emitted when indentation decreases
-LINE                = emitted before a recipe line
 TEXT       = recipe text, only matches in a recipe body
 ```
 
+grammar syntax
+--------------
+
+```
+|   alternation
+()  grouping
+_?  option (0 or 1 times)
+_*  repetition (0 or more times)
+_+  repetition (1 or more times)
+```
+
 grammar
 -------
 
@@ -45,11 +52,13 @@ assignment    : NAME '=' expression eol
 
 export        : 'export' assignment
 
-expression    : STRING
+expression    : value '+' expression
+              | value
+
+value         : STRING
               | RAW_STRING
               | NAME
               | BACKTICK
-              | expression '+' expression
 
 recipe        : '@'? NAME parameter* ('+' parameter)? ':' dependencies? body?