Fix subtle bug in PolyIndex and improve docstrings

1 files changed, 14 insertions, 6 deletions

diff --git a/mdpoly/types.py b/mdpoly/types.py
index 0a4c68e..b580b99 100644
--- a/mdpoly/types.py
+++ b/mdpoly/types.py
@@ -76,9 +76,17 @@ class PolyVarIndex(NamedTuple):
     @classmethod
     @property
     def constant(cls):
-        """ Special index for constants, which have no variable """
+        """ Special index for constants.
+
+        Constants do not have an associated variable,
+        and the field power is ignored.
+        """
         return cls(var_idx=-1, power=0)
 
+    @staticmethod
+    def is_constant(index: Self) -> bool:
+        return index.var_idx == -1
+
     @classmethod
     def of_var(cls, variable: Var, state: State, power: Number =1) -> Self:
         return cls(var_idx=state.index(variable), power=power)
@@ -90,17 +98,17 @@ class PolyIndex(tuple[PolyVarIndex]):
 
     Example: 
 
-        Suppose there are two variables x, y with indices 0, 1 respectively.
-        Then the monomal xy^2 has PolyIndex
+        Suppose there are two variables x, y with indices 0, 1 respectively (in
+        the State object). Then the monomal xy^2 has PolyIndex
 
-        (PolyVarIndex(varindex=0, power=1), PolyVarIndex(varindex=1, power=2)
+        (PolyVarIndex(var_idx=0, power=1), PolyVarIndex(var_idx=1, power=2)
 
         Then given the polynomial
 
             p(x, y) = 3x^2 + 5xy^2
 
         The with the PolyIndex above we can retrieve the coefficient 5 in front
-        of xy^2.
+        of xy^2 from the Repr object.
     """
 
     def __repr__(self) -> str:
@@ -112,4 +120,4 @@ class PolyIndex(tuple[PolyVarIndex]):
     @property
     def constant(cls):
         """ Index of the constant term """
-        return cls(PolyVarIndex.constant)
+        return cls((PolyVarIndex.constant,))