Question: How to select the indices in a table for which the entries are not empty?

Hello

After using maple for quite a while, I am still confused by some basic concepts.   

Given the following table, how can I select the indices for which the entry is not null ([])?  

table([1 = NULL, 2 = NULL, 3 = NULL, 4 = NULL, 5 = NULL, 6 = NULL, 7 = 5, 9 = NULL, 8 = NULL, 11 = 4, 10 = NULL, 13 = NULL, 12 = NULL, 15 = 9, 14 = NULL, 18 = NULL, 19 = 8, 16 = 9, 17 = NULL, 22 = 9, 23 = NULL, 20 = NULL, 21 = 8, 27 = NULL, 26 = 8, 25 = 4, 24 = NULL, 31 = NULL, 30 = 9, 29 = NULL, 28 = 9, 36 = NULL, 37 = 9, 38 = 9, 39 = NULL, 32 = 5, 33 = NULL, 34 = NULL, 35 = NULL, 45 = NULL, 44 = NULL, 47 = NULL, 46 = NULL, 41 = 8, 40 = NULL, 43 = NULL, 42 = NULL, 54 = NULL, 55 = NULL, 52 = NULL, 53 = NULL, 50 = NULL, 51 = NULL, 48 = 5, 49 = 9, 60 = 8, 59 = NULL, 58 = 7, 57 = 7, 56 = NULL])

Many thanks