code: public function configure(array $configuration = null)
      {
          parent::configure($configuration);
  
          if (isset($this->configuration['functions'])) {
              $this->functions = $this->configuration['functions'];
  
              return;
          }
  
          // assertions added in 3.0: assertArrayNotHasKey assertArrayHasKey assertFileNotExists assertFileExists assertNotNull, assertNull
          $this->functions = [
              'array_key_exists',
              'file_exists',
              'is_null',
          ];
  
          if (PhpUnitTargetVersion::fulfills($this->configuration['target'], PhpUnitTargetVersion::VERSION_3_5)) {
              // assertions added in 3.5: assertInternalType assertNotEmpty assertEmpty
              $this->functions = array_merge($this->functions, [
                  'empty',
                  'is_array',
                  'is_bool',
                  'is_boolean',
                  'is_callable',
                  'is_double',
                  'is_float',
                  'is_int',
                  'is_integer',
                  'is_long',
                  'is_numeric',
                  'is_object',
                  'is_real',
                  'is_resource',
                  'is_scalar',
                  'is_string',
              ]);
          }
  
          if (PhpUnitTargetVersion::fulfills($this->configuration['target'], PhpUnitTargetVersion::VERSION_5_0)) {
              // assertions added in 5.0: assertFinite assertInfinite assertNan
              $this->functions = array_merge($this->functions, [
                  'is_infinite',
                  'is_nan',
              ]);
          }
  
          if (PhpUnitTargetVersion::fulfills($this->configuration['target'], PhpUnitTargetVersion::VERSION_5_6)) {
              // assertions added in 5.6: assertDirectoryExists assertDirectoryNotExists assertIsReadable assertNotIsReadable assertIsWritable assertNotIsWritable
              $this->functions = array_merge($this->functions, [
                  'is_dir',
                  'is_readable',
                  'is_writable',
              ]);
          }
      }
docs: {@inheritdoc}