+// blends between two matrices, used primarily for animation interpolation
+// (note: it is recommended to follow this with Matrix4x4_Normalize, a method
+// known as nlerp rotation, often better for animation purposes than slerp)
+void Matrix4x4_Interpolate (matrix4x4_t *out, matrix4x4_t *in1, matrix4x4_t *in2, double frac);
+// zeros all matrix components, used with Matrix4x4_Accumulate
+void Matrix4x4_Clear (matrix4x4_t *out);
+// adds a weighted contribution from the supplied matrix, used to blend 3 or
+// more matrices with weighting, it is recommended that Matrix4x4_Normalize be
+// called afterward (a method known as nlerp rotation, often better for
+// animation purposes than slerp)
+void Matrix4x4_Accumulate (matrix4x4_t *out, matrix4x4_t *in, double weight);