diff --git a/src/Continuous/discretization.jl b/src/Continuous/discretization.jl
index a5da200e2..4770d0ff0 100644
--- a/src/Continuous/discretization.jl
+++ b/src/Continuous/discretization.jl
@@ -42,7 +42,7 @@ function discretize(ivp::IVP{<:CLCS, <:LazySet}, δ::Float64, alg::Forward)
     Phi2A_abs = Φ₂(A_abs, δ, alg.phi2_method)
 
     # "forward" algorithm, uses E⁺
-    @assert alg.sih_method == "concrete"
+    @assert alg.sih_method == :concrete
     # TODO : specialize, add option to compute the concrete linear map
     Einit = symmetric_interval_hull(Phi2A_abs * symmetric_interval_hull((A * A) * X0))
 
@@ -62,7 +62,7 @@ function discretize(ivp::IVP{<:CLCCS, <:LazySet}, δ::Float64, alg::Forward)
     A_abs = _elementwise_abs(A)
     Phi2A_abs = Φ₂(A_abs, δ, alg.phi2_method)
 
-    @assert alg.sih_method == "concrete"
+    @assert alg.sih_method == :concrete
     # TODO : specialize, add option to compute the concrete linear map
     Einit = symmetric_interval_hull(Phi2A_abs * symmetric_interval_hull((A * A) * X0))
 
diff --git a/src/Continuous/exponentiation.jl b/src/Continuous/exponentiation.jl
index c12976f20..c90822eab 100644
--- a/src/Continuous/exponentiation.jl
+++ b/src/Continuous/exponentiation.jl
@@ -20,7 +20,7 @@ using LinearAlgebra: checksquare
 @inline _exp(A::IdentityMultiple) = IdentityMultiple(exp(A.M.λ), size(A, 1))
 
 """
-    _exp(A::AbstractMatrix, δ::Float64, method::String)
+    _exp(A::AbstractMatrix, δ::Float64, method::Symbol)
 
 Compute the matrix exponential ``e^{Aδ}``.
 
@@ -28,15 +28,15 @@ Compute the matrix exponential ``e^{Aδ}``.
 
 - `A`       -- matrix
 - `δ`       -- step size
-- `method`  -- the method used to take the matrix exponential of `A`;
+- `method`  -- symbol with the method used to take the matrix exponential of `A`;
                possible options are:
 
-    - `"base"` -- use the scaling and squaring method implemented in Julia standard
-                  library; see `?exp` for details
-    - `"lazy"` -- return a lazy wrapper type around the matrix exponential using
-                  the implementation `LazySets.SparseMatrixExp`
-    - `"pade"` -- apply Pade approximant method to compute the matrix exponential
-                  of a sparse matrix (requires `Expokit`)
+    - `:base` -- use the scaling and squaring method implemented in Julia standard
+                 library; see `?exp` for details
+    - `:lazy` -- return a lazy wrapper type around the matrix exponential using
+                 the implementation `LazySets.SparseMatrixExp`
+    - `:pade` -- apply Pade approximant method to compute the matrix exponential
+                 of a sparse matrix (requires `Expokit`)
 
 ### Output
 
@@ -48,15 +48,15 @@ If the algorithm `"lazy"` is used, evaluations of the action of the matrix
 exponential are done with the `expmv` implementation from `Expokit`
 (but see `LazySets#1312` for the planned generalization to other backends).
 """
-function _exp(A::AbstractMatrix, δ::Float64, method::String)
+function _exp(A::AbstractMatrix, δ::Float64, method::Symbol)
     n = checksquare(A)
-    if method == "base"
+    if method == :base
         return _exp(A * δ) # TODO use dots ? (requires MathematicalSystems#189 for IdentityMultiple)
 
-    elseif method == "lazy"
+    elseif method == :lazy
         return _exp_lazy(A * δ)
 
-    elseif method == "pade"
+    elseif method == :pade
         @requires Expokit
         return _exp_pade(A * δ)
 
@@ -122,15 +122,15 @@ function Φ₁(A::AbstractMatrix, δ::Float64, method::String)
     n = checksquare(A)
     B = _Aδ_3n(A, δ, n)
 
-    if method == "base"
+    if method == :base
         P = _exp(B)
         return P[1:n, (n+1):2*n]
 
-    elseif method == "lazy"
+    elseif method == :lazy
         P = _exp_lazy(B)
         return sparse(get_columns(P, (n+1):2*n)[1:n, :])
 
-    elseif method == "pade"
+    elseif method == :pade
         P = _exp_pade(B)
         return P[1:n, (n+1):2*n]
 
@@ -185,23 +185,23 @@ It can be shown that `Φ₂(A, δ) = P[1:n, (2*n+1):3*n]`.
 International Conference on Computer Aided Verification. Springer, Berlin,
 Heidelberg, 2011.
 """
-function Φ₂(A::AbstractMatrix, δ::Float64, method::String)
-    if method == "inverse"
+function Φ₂(A::AbstractMatrix, δ::Float64, method::Symbol)
+    if method == :inverse
         return _Φ₂_inverse(A, δ)
     end
 
     n = checksquare(A)
     B = _Aδ_3n(A, δ, n)
 
-    if method == "base"
+    if method == :base
         P = _exp(B)
         return P[1:n, (2*n+1):3*n]
 
-    elseif method == "lazy"
+    elseif method == :lazy
         P = _exp_lazy(B)
         return sparse(get_columns(P, (2*n+1):3*n)[1:n, :])
 
-    elseif method == "pade"
+    elseif method == :pade
         P = _exp_pade(B)
         return P[1:n, (2*n+1):3*n]