[Xy-pic] half-solid ellipse

[Chris Brav]

hi. i'm trying to make 2D cobordism diagrams. i like to draw them from
left to right, with solid circles on the in direction (left-hand) and half-solid,
half-dotted circles on the out direction (right-hand), with the solid half
"out front" and the dotted half "behind". a. lauda's code does this for diagrams drawn
from top to bottom, but i can't get it to work from left to right.

here is my code:

\[ \xy
(7,0)*\ellipse(1,3){.};
(7,0)*\ellipse(1,3)__,=:a(270){-};
(0,2.5)*\ellipse(1,3){-};
(0,-2.5)*\ellipse(1,3){-};
(14,3)*{}="TR";
(14,-3)*{}="BR";
(0,8)*{}="TTL";
(0,2)*{}="TBL";
(0,-2)*{}="BTL";
(0,-8)*{}="BBL";
(10,3)*{}="A";
(10,-3)*{}="B";
"TTL";"A" **\crv{(5,8) & (3,4)};
"A";"TR" **\dir{-};
"BBL";"B" **\crv{(5,-8) & (3,-4)};
"B";"BR" **\dir{-};
"TBL";"BTL" **\crv{(3,2) & (3,-2)};
\endxy \]

the trouble is line #3. i presume =:a() is supposed to set an angle, but
no matter what angle i try, i can't get rid of the extra solid part
showing from "behind".

any suggestions?

chris brav