[chaley]

The easiest way to handle this situation is to use a prefix in the template. The prefix is added only if the field is not empty. For example, warning would be

{#warnings:|Warning: |}

and genre would be

{#genre:|Genre: |}

for more explanation, see Advanced formatting in the user manual.

Of course, it could be done with the test function. For example, the following examples in template program mode and general program mode work. However, unless the computation is more complicated than strcat, it isn't worth the trouble.

{#warnings:'test($, strcat('Warning: ', $), '')'}

or

program:
	v = field('#warnings');
	test(v, strcat('Warning: ', v), '')