Add EBNF grammar.

Author: Patrick Thomson

docs/core-grammar.md

@@ -0,0 +1,30 @@
+# Semantic Core grammar
+
+This is an EBNF grammar for the (experimental) core IR language.
+
+```
+	expr ::= expr '.' expr
+	       | expr ' '+ expr
+	       | '{' expr (';' expr)* ';'? '}'
+	       | 'if' expr 'then' expr 'else' expr
+	       | ('lexical' | 'import' | 'load') expr
+	       | lit
+	       | 'let'? lvalue '=' expr
+	       | '(' expr ')'
+
+	lvalue ::= ident
+	         | parens expr
+
+	lit ::= '#true'
+	      | '#false'
+	      | 'unit'
+	      | 'frame'
+	      | lambda
+	      | ident
+
+	lambda ::= ('λ' | '\') ident ('->' | '→') expr
+
+	ident ::= [A-z_] ([A-z0-9_])*
+	        | '#{' [^{}]+ '}'
+	        | '"' [^"]+ '"'
+	```