def close_to(other,epsilon=Float::EPSILON)
      other_value = @unit.convert(other)
      abs_sum = @value.abs+other_value.abs
      (@value-other_value).abs <= abs_sum*epsilon
    end
def close_to(other,epsilon=Float::EPSILON)
      other_value = @unit.convert(other)
      abs_sum = @value.abs+other_value.abs
      (@value-other_value).abs <= abs_sum*epsilon
    end
Comparison of quantities.
 Before the comparison, it converts +other+ to the unit of +self+.
 @param  [Phys::Quantity] other
 @return [Integer]
 @raise  [Phys::UnitError] if unit conversion is failed.
 Equality. Returns +true+ if +self+ has the same value as +other+.
 Before the comparison, it converts +other+ to the unit of +self+.
 @param  [Phys::Quantity] other
 @return [Boolean]
 Comparison. Returns +true+ if +self+ is greater-than or equal-to +other+.
 Before the comparison, it converts +other+ to the unit of +self+.
 @param  [Phys::Quantity] other
 @return [Boolean]
 @raise  [Phys::UnitError] if unit conversion is failed.
 Comparison. Returns +true+ if +self+ is less-than or equal-to +other+.
 Before the comparison, it converts +other+ to the unit of +self+.
 @param  [Phys::Quantity] other
 @return [Boolean]
 @raise  [Phys::UnitError] if unit conversion is failed.
 Comparison. Returns +true+ if +self+ is less than +other+.
 Before the comparison, it converts +other+ to the unit of +self+.
 @param  [Phys::Quantity] other
 @return [Boolean]
 @raise  [Phys::UnitError] if unit conversion is failed.
 Comparison. Returns +true+ if +self+ is greater than +other+.
 Before the comparison, it converts +other+ to the unit of +self+.
 @param  [Phys::Quantity] other
 @return [Boolean]
 @raise  [Phys::UnitError] if unit conversion is failed.
 Closeness. Returns +true+ if
   (self-other).abs <= (self.abs+other.abs) * epsilon
 Before the comparison, it converts +other+ to the unit of +self+.
 @param  [Phys::Quantity] other
 @param  [Numeric] epsilon
 @return [Boolean]
 @raise  [Phys::UnitError] if unit conversion is failed.