The use of the keyword undefined is now deprecated. Use nothing instead.

• Use returns nothing when defining a handler which returns no value.
• Use nothing to indicate no value when manipulating optionally type variables

The is defined, is undefined, is not defined, and is not undefined syntax is now deprecated. Use is and is not with nothing instead.

• Use <expr> is nothing and <expr> is not nothing to test whether an expression has a value or not
• The expression <left> is <right> will now evaluate to true if <left> and <right> are both nothing
• The expression <left> is not <right> will now evaluate to true if one of <left> or <right> are nothing (but not both).