curl --request POST \
  --url https://api.iotools.cloud/v1/tool/double-integral-calculator \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "f": "x*y",
  "a": "0",
  "b": "1",
  "c": "0",
  "d": "1"
}
'

{
  "tool": "double-integral-calculator",
  "tool_version": "1.0.2",
  "outputs": {
    "value": "0.25",
    "setup": "∫∫ f(x, y) dy dx, f(x, y) = x*y, x ∈ [0, 1], y ∈ [0, 1]",
    "steps": "f(x, y) = x*y\nRegion: x ∈ [0, 1], y ∈ [0, 1]\n∫∫ f(x, y) dy dx = 0.25\nMethod: composite 2D Simpson's rule, integrated over y first then x, 100×100 subdivisions (101×101 grid of points)."
  },
  "credits_used": 5,
  "credits_remaining": null
}