Result is -Number
    which should be preferred for portability.
Result is the negative of Number.
In coroutining mode, if Number is uninstantiated, the call to -/2 is delayed until this variable is instantiated.
    Result is - 1.		% gives Result = -1
    Result is - 5.        	% gives Result = -5
    Result is - -6.2.     	% gives Result = 6.2
    Result is -(1).		% gives Result = -1
    Result is -(5).        	% gives Result = -5
    Result is -(-6.2).     	% gives Result = 6.2