ValidMaskAt
uint32 ValidMaskAt(uint32 x, uint32 y) const;
validMask = field.ValidMaskAt(x, y);
uint32 validMask = step->Field()->ValidMaskAt(x, y);
uint32 validMask = fField->ValidMaskAt(x, y);
uint32 validMask = fField->ValidMaskAt(x, y);
uint32 validMask = fField->ValidMaskAt(x, y);