Fix backward matching

src/eko/evolution_operator/operator_matrix_element.py

@@ -14,6 +14,7 @@
 
 from .. import basis_rotation as br
 from .. import scale_variations as sv
+from ..io.types import InversionMethod
 from . import Operator, QuadKerBase
 
 logger = logging.getLogger(__name__)
@@ -31,7 +32,7 @@ def build_ome(A, matching_order, a_s, backward_method):
         perturbation matching order
     a_s : float
         strong coupling, needed only for the exact inverse
-    backward_method : ["exact", "expanded" or ""]
+    backward_method : [InversionMethod.EXACT, InversionMethod.EXPANDED or ""]
         empty or method for inverting the matching condition (exact or expanded)
 
     Returns
@@ -49,7 +50,7 @@ def build_ome(A, matching_order, a_s, backward_method):
     # .end
     ome = np.eye(len(A[0]), dtype=np.complex_)
     A = np.ascontiguousarray(A)
-    if backward_method == "expanded":
+    if backward_method is InversionMethod.EXPANDED:
         # expended inverse
         if matching_order[0] >= 1:
             ome -= a_s * A[0]
@@ -66,7 +67,7 @@ def build_ome(A, matching_order, a_s, backward_method):
         if matching_order[0] >= 3:
             ome += a_s**3 * A[2]
         # need inverse exact ?  so add the missing pieces
-        if backward_method == "exact":
+        if backward_method is InversionMethod.EXACT:
             ome = np.linalg.inv(ome)
     return ome
 
@@ -114,7 +115,7 @@ def quad_ker(
         number of active flavor below threshold
     L : float
         :math:``\ln(\mu_F^2 / m_h^2)``
-    backward_method : ["exact", "expanded" or ""]
+    backward_method : [InversionMethod.EXACT, InversionMethod.EXPANDED or ""]
         empty or method for inverting the matching condition (exact or expanded)
     is_msbar: bool
         add the |MSbar| contribution