addition of a second possible behavior of present

Author: jvdp1

proposals/default_optional_arguments/proposal.txt

@@ -106,9 +106,14 @@ compatibility.
 
 4.1 present()
 
-When the optional dummy argument has an initializer, the function
-present() will return .true., even if the actual argument is not passed
-by the caller.
+There are two possible behaviors of the function present() when an
+initializer is provided.
+
+The first behavior would be that the function present() always returns
+.true. when the optional dummy argument has an initializer, and
+independently of the fact that an actual argument is passed or not
+by the caller. Therefore, with such a behavior, the function present()
+becomes useless for optional arguments with a default value.
 
 Example:
 
@@ -120,6 +125,22 @@ Example:
       print*, present(b)  !always .true. due to the initializer
     end function foo
 
+The second behavior would be that the function present() returns
+.true. or .false. when an actual argument is passed or not by the
+caller, and independently of the fact that the optional dummy
+argument has, or hasn't an initializer.
+
+A use case of this behavior of the function present() could be:
+
+    real function foo(a, b)
+      real, intent(in) :: a
+      real, intent(in), optional :: b = 1.234
+      if (present(b)) then
+       ... !expensive input validation  here
+      end if
+      ...
+    end function foo
+
 
 5. Further discussion