Symbols are user-defined names for addresses, numbers or macros.
Their maximum significant length is 31 characters. They can be even
longer, but everything behind the first 31 characters is ignored.
Symbols may consist of letters, digits, '_' and '?' characters.
A symbol name must not start with a digit!
Upper and lower case letters are considered to be equivalent.
Note: Assembly language keywords must not be redefined as user symbols!
Is_this_really_a_SYMBOL_? is a legal symbol name!