sig
  type internal_format =
      GL_ALPHA
    | GL_ALPHA4
    | GL_ALPHA8
    | GL_ALPHA12
    | GL_ALPHA16
    | GL_COMPRESSED_ALPHA
    | GL_COMPRESSED_LUMINANCE
    | GL_COMPRESSED_LUMINANCE_ALPHA
    | GL_COMPRESSED_INTENSITY
    | GL_COMPRESSED_RGB
    | GL_COMPRESSED_RGBA
    | GL_DEPTH_COMPONENT
    | GL_DEPTH_COMPONENT16
    | GL_DEPTH_COMPONENT24
    | GL_DEPTH_COMPONENT32
    | GL_LUMINANCE
    | GL_LUMINANCE4
    | GL_LUMINANCE8
    | GL_LUMINANCE12
    | GL_LUMINANCE16
    | GL_LUMINANCE_ALPHA
    | GL_LUMINANCE4_ALPHA4
    | GL_LUMINANCE6_ALPHA2
    | GL_LUMINANCE8_ALPHA8
    | GL_LUMINANCE12_ALPHA4
    | GL_LUMINANCE12_ALPHA12
    | GL_LUMINANCE16_ALPHA16
    | GL_INTENSITY
    | GL_INTENSITY4
    | GL_INTENSITY8
    | GL_INTENSITY12
    | GL_INTENSITY16
    | GL_R3_G3_B2
    | GL_RGB
    | GL_RGB4
    | GL_RGB5
    | GL_RGB8
    | GL_RGB10
    | GL_RGB12
    | GL_RGB16
    | GL_RGBA
    | GL_RGBA2
    | GL_RGBA4
    | GL_RGB5_A1
    | GL_RGBA8
    | GL_RGB10_A2
    | GL_RGBA12
    | GL_RGBA16
    | GL_SLUMINANCE
    | GL_SLUMINANCE8
    | GL_SLUMINANCE_ALPHA
    | GL_SLUMINANCE8_ALPHA8
    | GL_SRGB
    | GL_SRGB8
    | GL_SRGB_ALPHA
    | GL_SRGB8_ALPHA8
end