Use '_' to indicate no parent phoneme table in the 'phonemetable' command.

docs/phontab.md

 
 \<parent\> Is the name of a previously defined phoneme table whose phoneme
 definitions are inherited by this one. The name `base` indicates the first
-(base) phoneme table.
+(base) phoneme table. The name `_` indicates no parent phoneme table.
 
 ## Phoneme Definitions
 

src/libespeak-ng/compiledata.c

 				break;
 			}
 		}
-		if (ix == n_phoneme_tabs)
+		if (ix == n_phoneme_tabs && strcmp(item_string, "_") != 0)
 			error("Can't find base phonemetable '%s'", item_string);
 	} else
 		ReservePhCodes();