The SOR$BEGIN_MERGE routine initializes the merge operation by
    opening the input and output files and by providing the number of
    input files, the key specifications, and the merge options.

    Format

      SOR$BEGIN_MERGE  [key-buffer] [,lrl] [,options] [,merge_order]

                       [,user_compare] [,user_equal] [,user_input]

                       [,context]