Overload missing interface similarterm

Fix #699

Author: YingboMa

src/context_dsl.jl

@@ -6,6 +6,7 @@ isparameter(::Sym{<:Parameter}) = true
 isparameter(::Sym{<:FnType{<:Any, <:Parameter}}) = true
 
 SymbolicUtils.symtype(s::Symbolic{Parameter{T}}) where T = T
+SymbolicUtils.similarterm(t::Term{T}, f, args) where {T<:Parameter} = Term{T}(f, args)
 
 Base.convert(::Type{Num}, x::Symbolic{Parameter{T}}) where {T<:Number} = Num(x)
 

test/simplify.jl

@@ -32,3 +32,16 @@ d3 = Differential(x)(d2)
 
 @test ModelingToolkit.substitute(2x + y == 1, Dict(x => 0.0, y => 0.0)) === false
 @test ModelingToolkit.substitute(2x + y == 1, Dict(x => 0.0, y => 1.0)) === true
+
+# 699
+using SymbolicUtils: to_symbolic, substitute
+@parameters t a(t) b(t)
+
+# back and forth substitution does not work for parameters with dependencies
+term = to_symbolic(a)
+term2 = substitute(term, a=>b)
+@test term2 isa Term{ModelingToolkit.Parameter{Real}}
+@test isequal(term2, b)
+term3 = substitute(term2, b=>a)
+@test term3 isa Term{ModelingToolkit.Parameter{Real}}
+@test isequal(term3, a)