mekso n-ary operator: append covariant (lower) indices to tensor

Terminated by te'oi'oi. Takes ordered input (A, X_1, X_2, …, X_(n-1)), where A is a tensor and X_i is an index with an understood (or elsewhere defined) ordered domain. It appends/assigns X_i to be the ith covariant index of A, which is usually designated as a subscript in the ith position to the right of A. See also: ga'u'au. It is probable that Einstein summation notation will be conventionally in effect.