make literal parameter allowable in LHS timescale

Project.toml

@@ -1,7 +1,7 @@
 name = "ProcessBasedModelling"
 uuid = "ca969041-2cf3-4b10-bc21-86f4417093eb"
 authors = ["Datseris <datseris.george@gmail.com>"]
-version = "1.2.0"
+version = "1.2.1"
 
 [deps]
 ModelingToolkit = "961ee093-0014-501f-94e3-6117800e7a78"

src/API.jl

@@ -12,7 +12,8 @@ The type must extend the following functions from the module `ProcessBasedModell
 - (optional) `lhs(p)` which returns the left-hand-side. Let `τ = timescale(p)`.
   Then default `lhs(p)` behaviour depends on `τ` as follows:
   - Just `lhs_variable(p)` if `τ == NoTimeDerivative()`.
-  - `Differential(t)(p)` if `τ == nothing`.
+  - `Differential(t)(p)` if `τ == nothing`, or multiplied with a number
+    if `τ isa LiteralParameter`.
   - `τ_var*Differential(t)(p)` if `τ isa Union{Real, Num}`. If real,
     a new named parameter `τ_var` is created that has the prefix `:τ_` and then the
     lhs-variable name and has default value `τ`. Else if `Num`, `τ_var = τ` as given.
@@ -64,6 +65,8 @@ function lhs(p::Process)
         return D(v) # `D` is the MTK canonical variable for time derivative
     elseif τ isa NoTimeDerivative || iszero(τ) # no time variability
         return v
+    elseif τ isa LiteralParameter
+        return τ.p*D(v)
     else # τ is either Num or Real
         τvar = new_derived_named_parameter(v, τ, "τ")
         return τvar*D(v)