<formula> ::= [<number>] <sequence> { '+' [<number>] <sequence> } 
<sequence> ::= <element> [<number>] { <element> [<number>] } 
<element> ::= <chem> | '(' <sequence> ')' 
<chem> ::= <uppercase_letter> [ <lowercase_letter> ] 
<uppercase_letter> ::= 'A'..'Z' 
<lowercase_letter> ::= 'a'..'z' 
<number> ::= '1'..'9' { '0'..'9' }