Add a bracketed scalar rootfind node, kept internal for now

packages/pybamm/src/pybamm/__init__.py

@@ -32,6 +32,7 @@
 from .expression_tree.broadcasts import *
 from .expression_tree.functions import *
 from .expression_tree.conditional import Conditional
+from .expression_tree.brent import _Brent, _BrentUnknown
 from .expression_tree.interpolant import Interpolant
 from .expression_tree.discrete_time_sum import *
 from .expression_tree.input_parameter import InputParameter

packages/pybamm/src/pybamm/discretisations/discretisation.py

@@ -1148,7 +1148,11 @@ def _process_symbol(self, symbol):
                 else:
                     return symbol.create_copy(new_children=[disc_child])
 
-        elif isinstance(symbol, (pybamm.Function, pybamm.Conditional)):
+        elif isinstance(symbol, pybamm._BrentUnknown):
+            # bound by its Brent, so it has no state-vector slice to resolve
+            return symbol.create_copy()
+
+        elif isinstance(symbol, (pybamm.Function, pybamm.Conditional, pybamm._Brent)):
             disc_children = [self.process_symbol(child) for child in symbol.children]
             return symbol.create_copy(disc_children)