HP 48 SYSEVAL

[Giuseppe Donnini]

The fact that the built-in command-line parser is bypassed.

The operating system itself is able to accept any character within an identifier object: a space, a newline, a null character, whatever. It can also handle names that are identical to built-in command names, like DUP or SIN. The "rules" you refer to are a set of restrictions implemented at the end-user level in order to protect said user from himself, with the command-line parser being one mesh of the safety net.