\usepackage{xparse,booktabs}
\ExplSyntaxOn
\NewDocumentCommand{\colmult}{mm}
{
\colmult_do:nn { #1 } { #2 }
}
\tl_new:N \l_colmult_spaces_tl
\tl_new:N \l_colmult_partial_summands_tl
\seq_new:N \l_colmult_multiplier_seq
\cs_new_protected:Npn \colmult_do:nn #1 #2
{
\tl_clear:N \l_colmult_partial_summands_tl
\tl_clear:N \l_colmult_spaces_tl
\seq_set_split:Nnn \l_colmult_multiplier_seq { } { #2 }
\seq_reverse:N \l_colmult_multiplier_seq
\seq_map_inline:Nn \l_colmult_multiplier_seq
{