binary mekso operator: for ordered list X1, this word outputs the same ordered list except the indices/subscripts have been relabelled/redefined/reindexed according to rule X2 (see notes).
Default behavior for X2: (1) if the least index of X1 is less than or equal to 0, then the term which is so indexed is mapped to the term which is indexed by 1 and each subsequent entry of X1 is mapped so as to have the successor index of the immediately previous term (without any gaps); (2) if the least index of X1 is positive, then the term which is so indexed is mapped to the term which is indexed by 0 and each subsequent entry of X1 is mapped so as to have the successor index of the immediately previous term (without any gaps); thus the indexing is restricted to nonnegative intergers, is compressed, and toggles between starting at 1 and 0 respectively. Other behaviors may be specified. The cultural default starting index for a list is determined by context.