Possible error in Shadows15.glslib shadowGather

 #ifdef GL_ARB_gpu_shader5
        vec4 coord = vec4(projCoord.xyz / projCoord.www,0.0);
        vec4 gather = SHADOWGATHER(tex, coord);
        vec4 gather = vec4(0.0);
        gather.x = SHADOWCOMPAREOFFSET(tex, projCoord, ivec2(0, 0));
        gather.y = SHADOWCOMPAREOFFSET(tex, projCoord, ivec2(1, 0));
        gather.z = SHADOWCOMPAREOFFSET(tex, projCoord, ivec2(0, 1));
        gather.w = SHADOWCOMPAREOFFSET(tex, projCoord, ivec2(1, 1));

   vec2 f = fract( projCoord.xy * SHADOWMAP_SIZE );
   vec2 mx = mix( gather.xz, gather.yw, f.x );
   return mix( mx.x, mx.y, f.y );

According to the spec (https://www.opengl.org/registry/specs/ARB/texture_gather.txt) and my experiments, textureGather has (0,1),(1,1),(1,0),(0,0) offsets in xyzw, not what is implied above. In particular, it will never make sense to interpolate between xz and yw based on single coordinate, as these values are in corners.

Thanks, gonna change it