{
 "id": "06-symmetric-extrude",
 "name": "Symmetric extrude",
 "description": "50x20 rectangle centred on origin on the XZ plane, extruded symmetrically 15 mm each side (30 total).",
 "exercises": [
  "sketch_xz",
  "extrude",
  "symmetric"
 ],
 "units": {
  "length": "mm",
  "angle": "deg",
  "area": "mm2",
  "volume": "mm3"
 },
 "conventions": {
  "world": "right-handed; Z up. XY/XZ/YZ are the origin planes.",
  "sketch_frame": "sketch-local 2D coords; world = plane.origin + x*plane.x_dir + y*plane.y_dir (origin in mm). Profile 'loops' list sketch entity ids.",
  "point_refs": "'<id>.start' / '<id>.end' / '<id>.center' reuse (share) that existing sketch point; 'at' is its position when referenced.",
  "extrude_direction": "positive = along sketch plane normal unless stated",
  "selections": "selected faces/edges carry 'point': a world-space point on the face / the edge's parametric midpoint; pick the nearest face/edge to it."
 },
 "parameters": [],
 "features": [
  {
   "op": "sketch",
   "name": "Sketch1",
   "plane": {
    "ref": "XZ",
    "origin": [
     0.0,
     0.0,
     0.0
    ],
    "x_dir": [
     1.0,
     0.0,
     0.0
    ],
    "y_dir": [
     0.0,
     0.0,
     -1.0
    ],
    "normal": [
     0.0,
     1.0,
     0.0
    ]
   },
   "entities": [
    {
     "type": "line",
     "start": {
      "at": [
       -25,
       -10
      ]
     },
     "end": {
      "at": [
       25,
       -10
      ]
     },
     "id": "l1"
    },
    {
     "type": "line",
     "start": {
      "ref": "l1.end",
      "at": [
       25.0,
       -10.0
      ]
     },
     "end": {
      "at": [
       25,
       10
      ]
     },
     "id": "l2"
    },
    {
     "type": "line",
     "start": {
      "ref": "l2.end",
      "at": [
       25.0,
       10.0
      ]
     },
     "end": {
      "at": [
       -25,
       10
      ]
     },
     "id": "l3"
    },
    {
     "type": "line",
     "start": {
      "ref": "l3.end",
      "at": [
       -25.0,
       10.0
      ]
     },
     "end": {
      "ref": "l1.start",
      "at": [
       -25.0,
       -10.0
      ]
     },
     "id": "l4"
    }
   ],
   "constraints": [],
   "dimensions": []
  },
  {
   "op": "extrude",
   "name": "Extrude1",
   "operation": "new",
   "profiles": [
    {
     "sketch": "Sketch1",
     "loops": [
      {
       "outer": true,
       "curves": [
        "l1",
        "l2",
        "l3",
        "l4"
       ]
      }
     ]
    }
   ],
   "extent": {
    "type": "symmetric",
    "distance": 15,
    "is_full_length": false,
    "meaning": "distance on each side"
   },
   "result_bodies": [
    "Block"
   ]
  }
 ]
}