How can I make html.table(data, header = ["header 1",...,"header n"]) generate the COLSPAN attribute in the header line?

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I love html.table(data, header = ["header 1",...,"header n"]) but I cannot figure out right now how to make it generate the html COLSPAN attribute when I want a cell of my header line spread across several columns.

E.g. consider a table of values of the functions f(x,y,z), g(x,y,z), h(x,y,z). The table has 3 columns with headers x, y, z and 3 columns with headers f, g, h. How can I add another header line at the top with titles "variables" and "values" that both spread across 3 columns?