Add syntax file

syntax.ebnf

@@ -1 +1,24 @@
-digit: "0" | "1"
+digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
+
+letter = "a" | "b" | "c" | "d" | "e" | "f" | "g"
+       | "h" | "i" | "j" | "k" | "l" | "m" | "n"
+       | "o" | "p" | "q" | "r" | "s" | "t" | "u"
+       | "v" | "w" | "x" | "y" | "z" ;
+
+number = [ "-" ] , { digit }+ , [ "." { digit }+ ] ;
+
+identifier = { letter }+ ;
+
+operator = "*" | "/" | "%"  | "+"  | "-"  | "<<" | ">>"
+         | "<" | ">" | "<=" | ">=" | "==" | "!=" | "&"
+         | "^" | "|" | "&&" | "||"
+
+constant = "pi" | "e" | "tau" | "true" | "false"
+
+function = identifier, "(" param list ")"
+
+param list = ( ) | ( operand { ",", operand } )
+
+operand = number | constant | function
+
+expression = { operand, operator }, operand