Generate the Stöhr sequence I am learning Ruby and wrote my first nontrivial code to solve this problem.
The challenge is to generate the first n elements of the Stöhr sequence, S, which is defined as follows:

S[0] = 1
S[n] is the smallest number that cannot be expressed as the sum of two distinct previous elements in the sequence.

Thus the sequence begins with 1, 2, 4, 7, and 10. The next element is 13, because 11 (=1+10) and 12 (=2+10) are sums of previous elements, but 13 is not.
I am looking for the shortest code. My own, in Ruby, is 108 characters long, but maybe I'll wait to see what others come up with before posting it?
APL, 7

In APL you can choose if you want to work with index 0 or index 1.
You do this by setting global variable ⎕IO←0

If we choose to work in index 0 we have:

+\3⌊1⌈⍳


Explanation: 

⍳    creates a sequence 0...n   (0 1 2 3 4 5)
1⌈   takes whichever is bigger, number in sequence or 1 (1 1 2 3 4 5)
3⌊   takes whichever is lower, number in sequence or 3 (1 1 2 3 3 3)
+\   partial sums for the sequence (1 2 4 7 10 13)


Try it on tryapl.org