miser

import std.math : PI, pow;
import std.stdio : writeln;
auto func = function( double[] xs )
{
    if (pow(xs[0],2)+pow(xs[1],2)<= 1.0)
        return 1.0;
    return 0.0;
};

auto result = integrate( func, [-1.0,-1], [1.0,1.0], 1e-5, 0 );
result.writeln;
assert( result.value <= PI + 1e-2 );
assert( result.value >= PI - 1e-2 );