Operators Questions

[Richard Irvine <Richard.Irvine@eurocontrol.fr>]

Ada allows overloading of the predefined operators
but not definition of new operators.
Sometimes code could be made more readable if it
were possible to define new operators, e.g.

if   aTimeInterval encloses anotherTimeInterval 
then -- It is clear which interval encloses which.
     ...

instead of

if   encloses(aTimeInterval, anotherTimeInterval)
then -- Which interval enclosed which?
     -- One has to look at the declaration of
     -- the function.
     -- One could use formal parameters, but
     -- then things start to get long-winded.
     ...

I guess that allowing the programmer to define his
own operators creates difficulties for the compiler
writer, or is there some other reason why definition
of new operators is not allowed in Ada? 

Are there languages which do allow defintion of 
new operators?