The definition of the variable
< number>::=< digit > | < digit >< number >
where < digit > is defined as
< digit > ::= 1|2|3|4|5
Apparently reflects the following syntax diagram
Please explain why this is the case. I am particularly confused with the clause after the vertical OR line (i.e. < digit >< number >) and whether it has something to do with the fact that number can consist of many digits.