This function moves the iteration pointer backward one element.
If the iteration pointer is not defined the use of the function will raise an error hash::ErrorNoThisKey. The constant for the error code is defined in the module file `hash.bas'.
The return value of the function is the key of the element pointed by the iteration pointer after the step or undef if the iteration passed the first element. Note that the key may also be undef, therefore checking the return value of this function can only be used in situation when the undef value is guaranteed not be used as key and the hash is not empty. To check the end of an iteration use the function hash::Exists.
For more information on iteration over hash elements see the section start