input is this (it's actually a list, but dict makes it easier to see where 0, 1, 2 are coming from 
{0:(1870,1900), 1:(1870,1950), 2: (1930,1960)}

output is this (.. is 'range') 
1870..1899: (0, 1)
1900..1929: (1)
1930..1949: (1, 2)
1950..1959: (2)