Three extension are common to most EBNFs

• The first extension denotes an optional part of a RHS, which is delimited by brackets. For example:   -> if ( ) statement [ else ]
• The second extension is the use of braces in an RHS to indicate that the enclosed part can be repeated indefinitely or left out altogether. For example:  -> identifier {, }
• The third extension deals with multiple choice options. For example:  -> term ( *| / | % )