Gap finder

There is some database-relation called "t". It's know that the result of

SELECT x FROM t;

has following form:

a
a+1
a+2
...
a+k1-1
a+k1+1
...
a+k2-1
a+k2+1
...
a+kn-1
a+kn+1
...
a+b-1
a+b

Mind, that k_i + 1 < k_i+1.

Write SQL query that returns:

a+k1
a+k2
...
a+kn.

Don't use cool features like Postgres generate_series.