My BNF so far

bnf.md

statement       => expression* | return ; // initially only expressions and a final return is supported (not sure how to represent it)
return          => "return" low_bin ";"? ;
expression      => assignment | reassignment; 

assignment      => ("const" | "mut") IDENTIFIER ( "=" ) low_bin ";"? ;
reassignment    => IDENTIFIER ( "=" | "/=" | "-=" | "+=" | "*=" | "&=" | "^=" | "|=" ) low_bin ";"? ;
low_bin         => high_bin ( ( "-" | "+") high_bin )* ;
high_bin        => TERMINAL ( ( "/" | "*" | "%" ) TERMINAL )* ;

TERMINAL        => NUMBER | IDENTIFIER ;
NUMBER          => DIGIT+ ( "." DIGIT+ )? ;
IDENTIFIER      => < any ALPHA except "_" > ( ALPHA | DIGIT )* ;