The definition of the variable <number> < number>::=< digit > | < digit >< number > where < digit > is defined as < digit > ::= 1|2|3|4|5 Apparently reflects the following syntax diagram [![enter image description here][1]][1] [1]: https://i.sstatic.net/7C6zs.jpg 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.