Add method empty() to the API of Population (#268)

And make existing `__bool__()` method use it.

grammarinator/runtime/population.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2023-2024 Renata Hodovan, Akos Kiss.
+# Copyright (c) 2023-2025 Renata Hodovan, Akos Kiss.
 #
 # Licensed under the BSD 3-Clause License
 # <LICENSE.rst or https://opensource.org/licenses/BSD-3-Clause>.
@@ -14,17 +14,26 @@ class Population:
     individuals) and can select trees for mutation or recombination based on some strategy.
     """
 
-    def __bool__(self):
+    def empty(self):
         """
-        Truth value testing of Populations.
+        Return whether the population is empty.
 
         Raises :exc:`NotImplementedError` by default.
 
-        :return: ``True`` if the population is not empty and ``False`` otherwise.
+        :return: ``True`` if the population is empty and ``False`` otherwise.
         :rtype: bool
         """
         raise NotImplementedError()
 
+    def __bool__(self):
+        """
+        Truth value testing of Populations.
+
+        :return: ``True`` if the population is not empty and ``False`` otherwise.
+        :rtype: bool
+        """
+        return not self.empty()
+
     def add_individual(self, root, path=None):
         """
         Add a tree to the population.

grammarinator/tool/default_population.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2023-2024 Renata Hodovan, Akos Kiss.
+# Copyright (c) 2023-2025 Renata Hodovan, Akos Kiss.
 #
 # Licensed under the BSD 3-Clause License
 # <LICENSE.rst or https://opensource.org/licenses/BSD-3-Clause>.
@@ -39,11 +39,11 @@ def __init__(self, directory, extension, codec=None):
         os.makedirs(directory, exist_ok=True)
         self._files = glob.glob(join(self._directory, f'*.{self._extension}'))
 
-    def __bool__(self):
+    def empty(self):
         """
-        Check whether there is at least a single individual in the population.
+        Check whether the population contains no individuals.
         """
-        return len(self._files) > 0
+        return len(self._files) == 0
 
     def add_individual(self, root, path=None):
         """