<pre>
A, B, C, etc. refers to people.

1||2 refers to the two sides of the first condom.
3||4 refers to the two sides of the second condom.
etc.

(a) two condoms
<pre>
    A, B <code> men; C, D </code> women.
</pre>
1||2 3||4     A has sex with C.                                1   4
1||2         A has sex with D.                                1   4 2
3||4 1||2     B has sex with C.                                1 3 4 2
3||4         B has sex with D.                                1 3 4 2

(b) two condoms
<pre>
    A <code> man; B, C, D </code> women.                                A B C D
</pre>
1||2 3||4     A has sex with B.                                1 4
1||2         A has sex with C.                                1 4 2
1||2 4||3     A has sex with D.                                1 4 2 3

<pre>
    A, B, C <code> men; D </code> woman.
</pre>
1||2 3||4     A has sex with D.                                1     4
3||4         B has sex with D.                                1 3   4
2||1 3||4     C has sex with D.                                1 3 2 4

(c) two condoms
<pre>
    A, B, C = men.
</pre>
1||2 3||4     A has sex with B.                                1 4
4||3 2||1     B has sex with A.                                1 4
3||4         C has sex with B.                                1 4 3
4||3         B has sex with C.                                1 4 3
1||2 4||3     A has sex with C.                                1 4 3
3||4 2||1     C has sex with A.                                1 4 3

(d) k+1 condoms
<pre>
    A <code> woman; B, C, D, ... </code> men
</pre>
1||2         A has sex with B.                                1 2
1||2 4||3     A has sex with C.                                1 2 3
1||2 3||4     A has sex with D.                                1 2 3 4
and so on, after the first man there is one condom used for every two men

(e) ceiling(m/2 + 2n/3)
see Ilan Vardi, Computational Recreations in Mathematica, Addison Wesley,
1991, p. 205
</pre>
