static _traceAmplitudeProbabilitySquare(trace, real, imag, x, y, d) {
        let p = real*real + imag*imag;
        if (p > 0.001) {
            trace.polygon([
                x, y + d * (1 - p),
                x + d, y + d * (1 - p),
                x + d, y + d,
                x, y + d]);
        }
    }
@param {!Tracer} trace
@param {!number} real
@param {!number} imag
@param {!number} x
@param {!number} y
@param {!number} d
@private