resolve single line with multiple items into mutliple lines, single items

Old line(columns tab-delimited):

Col1 Col2 Col3 ... Coln
A B1@B2 C ... N1@N2@N3

New lines
A B1 C .. N1
A B1 C .. N2
A B1 C .. N3
A B2 C .. N1
A B2 C .. N2
A B2 C .. N3

The problem is: although pattern matching can recognize "@", but how to
write the code generically so to get all N1, N2 and N3, such that the number
of items aren't known beforehand?


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