#macro sawhorse_strap(
  open_ang, is_open,
  wood_wd, wood_th,
  tape_wd, tape_th,
  what
)
  // One half-strap of the sawhorse.
  // It fits in the notch of the bottom cross beam.
  // The cylinder that forms the rounded-off part of that notch is "the knee".
  // {free_len} length of the unsupported part of the half-strap when open.
  // {open_ang} max opening angle, in degrees.
  // {is_open}  true iff the sawhorse is open.
  // {wood_wd}  width of wood beams.
  // {wood_th}  thickness of wood beams.
  // {knee_rad} radius of knee.
  // {tape_wd}  width of tape.
  // {tape_th}  tape thickness.
  
  ??? Revise below
  // Notch on cross-beam for strap.
  // The cylinder that makes up the rounded part at top  is the "knee".
  #local sh_wd = tape_wd + 2.0;         // Width of strap notch.
  #local sh_dp = 3;                     // Depth of strap notch on outer side.
  #local sh_rd = 0.5*(wood_th - sh_dp); // Radius of strap notch.
  #local sh_cx = -0.5*wood_th + sh_rd;  // X of center of strap notch knee.
  #local sh_dy = round_to_half( strap_cy - 0.5*sh_wd ); // Y of outer edge of strap notch.
  #debug concat( "\n!! strap notch dy = ", str(sh_dy,5,1), "\n" )
  #debug concat( "\n!! strap notch dp = ", str(sh_dp,3,1), "\n" )
  #debug concat( "\n!! strap notch wd = ", str(sh_wd,4,0), "\n" )

  ??? Revise below to account for displacement of half-horse in 
  ??? the +X direction by {hinge_ww/2}
      
  // Straps:
  #local strap_aa = bot_cross_ht + wood_wd - sh_rd; // Z pos of center of notch cyl, closed.
  #local strap_bb = closed_ht - hinge_rd - strap_aa; // Z dist from hinge to ctr ofnotch cyl, closed.
  #local strap_cc = wood_th + sh_rd; // X pos of ctr of notch cyl, closed.
  #local strap_free_len = strap_bb*sang + strap_cc*cang; // Free length of strap.

  // Z of strap (top of strap notch knee)
  #if (is_open)
    #local strap_dz = strap_aa * cang + strap_cc*sang + sh_rd; // Z disp of strap.
  #else
    #local strap_dz = strap_aa + sh_rd;
  #end
  

  // The strap is centered in Y with the top of the round part of the notch  
  // at Z = 0.
    
  #local eps = 0.01;  // Fudge displacement to avoid coincindent surfaces.
  #local eps3 = < eps, eps, eps >;  // Fudge 3D.

  // Cosine and sine of opening half-angle:
  #local cang = cos( radians( open_ang/2 ) );
  #local sang = sin( radians( open_ang/2 ) );
  
  #local knee_cz = - knee_rad; // Z of knee axis in final strap.

  // Part of tape that wounds around knee, full round:
  #local small_bend_cyl = 
    difference{
      cylinder{
        < 0, -tape_wd/2 + eps, 0 >, 
        < 0, +tape_wd/2- eps, 0 >, 
        knee_rad + tape_th - eps 
      }
      cylinder{
        < 0, -tape_wd/2 - eps, 0 >,
        < 0, +tape_wd/2 + eps, 0 >,
        knee_rad + eps
      }
    }
    
  // Part of tape fixed to the bottom cross beam, vertical:
  #local bound_part_vert = 
    box{
      < 0 + eps,       -tape_wd/2 + eps, -(wood_wd - knee_rad) + eps >,
      < tape_th - eps, +tape_wd/2 - eps, 0 + eps >
      translate knee_rad*x
    }

  // Defines strap pieces:
  // {knee_cx}     X of knee axis.
  // {small_bend}  part of tape wrapped over knee.
  // {free_part}   unsupported part of tape.
  // The last two have the Y axis as the axis of the knee.
  #if (is_open)
    #local knee_cx = free_len;
    #local bound_part = object{ bound_part_vert rotate -(open_ang/2)*y }
    #local small_bend = 
      difference{
        object{ small_bend_cyl }
        box{
          < -wood_th, -tape_wd, -wood_th >,
          < +wood_th, +tape_wd, 0 - eps  >
          rotate 0.5*open_ang*y
        }
        box{
          < -wood_th, -tape_wd, -wood_th >,
          < 0 + eps,  +tape_wd, +wood_th  >
        }
      }
    #local free_part = 
      box{
        < -free_len - eps,  -tape_wd/2 + eps,  0 + eps >,
        < 0 + eps,           +tape_wd/2 - eps, tape_th - eps >
        translate knee_rad*z
      }
  #else
    #local knee_cx = wood_th + knee_rad;
    #local bound_part = object{ bound_part_vert }
    #local small_bend = 
      difference{
        object{ small_bend_cyl }
        box{
          < -wood_th, -tape_wd, -wood_th >,
          < +wood_th, +tape_wd, 0 - eps  >
        }
      }
    // Extra length of tape wrapped around knee:
    #local small_extra_len = (knee_rad + tape_th/2)*radians(90 - open_ang/2);
    // Bottom bend, full round, centered on Y axis:
    #local bot_bend_cyl = 
      difference{
        cylinder{
          < 0, -tape_wd/2 + eps, 0 >, 
          < 0, +tape_wd/2- eps, 0 >, 
          wood_th - eps 
        }
        cylinder{
          < 0, -tape_wd/2 - eps, 0 >,
          < 0, +tape_wd/2 + eps, 0 >,
          wood_th - tape_th + eps
        }
      }
    // Length of bottom half-loop:
    #local bot_bend_len = (wood_th - tape_th/2)*radians(90);
    // Bottom half-loop:
    #local bot_bend = 
      intersection{
        object{ bot_bend_cyl }
        box{ 
          < 0 - eps,   -tape_wd, -2*wood_th >, 
          < 2*wood_th, +tape_wd, 0 + eps >
        }
      }
    // length of straight dangling part:
    #local free_part_ht = free_len - small_extra_len- bot_bend_len;
    #local free_part = 
      union{
        object{ bot_bend
          translate -(wood_th + knee_rad)* x
          translate -free_part_ht*z
        }
        box{
          < -tape_th + eps,  -tape_wd/2 + eps, -free_part_ht - eps >,
          < 0 - eps,         +tape_wd/2 - eps, 0 + eps >
          translate -knee_rad*x
        }
      }
  #end
  
  #local obj = 
    union{
      object{ bound_part }
      object{ small_bend }
      object{ free_part }
      translate < knee_cx, 0, knee_cz >
      texture{ tx_tape }
    }
  
  obj
#end