latex - How to get one component of a tikz/PGF coordinate? -

I am trying to draw a horizontal line in my diagram. Y coordinate of the line should be between halfway between one and B (A below B). The left and right end points of the line are on the bounding box of tikzpicture using the intersection operator, how am I doing now:

 \ coordin (h0) ($ (a.north)! 0.5 ! (B.south) $); \ Draw (h0 - | Current Bounding Box. West) - (h0 - | Current Bounding Box.Ast); 

It kills me as an intersection, whatever I want to do, it meets the coordinates of HK (HK) and X coordinates of east and west edges of the bounding box, and write the coordinates themselves I do this But this syntax is not supported:

 \ coordinates (h0) ($ (a.north)! 0.5! (B.south) $); \ Draw (current bounding box.wexx, hl) - (current bounding box. East.x, h0i); 

Is there a way to refer to the individual components of the coordinate I 'M' is missing?

You can get inside the let operation in the components. Look for work in PGF manual, but by memory:

  \ draw \ p1 = ($ (a.north)! 0.5! (B.south) $), \ P2 = (current (\ X2, \ y1) - (\ x3, \ y1); in the bounding box. West), \ p3 = (the current bounding box.  

Maybe that would require debugging ... Edit: And now thanks for the questioner.