Unary operator after binary operator: legal or not?

["Jeffrey R. Carter" <spam.jrcarter.not@acm.nospam.org>]

Here's something confusing I encountered:

with Ada.Strings.Unbounded;

function Xyz (Left, Right : in Ada.Strings.Unbounded.Unbounded_String)
return String is
    function "+" (Right : in Ada.Strings.Unbounded.Unbounded_String)
    return String renames Ada.Strings.Unbounded.To_String;
begin -- Xyz
    if +Left <= +Right then
       return +Left & +Right;
       --            ^ Error reported here.
    end if;
end Xyz;

A compiler reports "missing operand", referencing the space after the "&".

Why then does it accept the comparison in the previous line? Both are

+Left [binary operator] +Right

-- 
Jeff Carter
"Have you gone berserk? Can't you see that that man is a ni?"
Blazing Saddles
38