Add a note to SpikeMixture logdensity implementation

Author: cscherrer

src/combinators/spikemixture.jl

@@ -23,6 +23,14 @@ end
 
 for func in [:logdensityof, :logdensity_def]
     @eval @inline function $func(μ::SpikeMixture, x)
+        # NOTE: We could instead write this as 
+        # R1 = typeof(log(one(μ.s))) 
+        # R2 = typeof(log(one(μ.w))) 
+
+        # which would rely on constant propagation insteadof type inference.
+        # We'll try this for now and come back to the question if we see
+        # problems.
+
         R1 = Core.Compiler.return_type(log, Tuple{typeof(μ.s)})
         R2 = Core.Compiler.return_type(log, Tuple{typeof(μ.w)})
         R3 = Core.Compiler.return_type($func, Tuple{typeof(μ.m),typeof(x)})