⍪ Catenate r←x⍪y

x and y, after extension, must have the same trailing axes (that is, the only difference in shape is the first element). Extension is as follows:

• if x and y have the same rank, no operation is performed
• if one of the arguments has rank one less than the other, the argument with lesser rank is promoted to have a 1-length leading axis
• if one of the arguments is scalar, it is reshaped to an array with leading axis of length 1 and trailing axes of the other argument
• if both arguments are scalar, they are reshaped to 1-element vectors.

After extension, r is the major cells of x followed by the major cells of y.