fix rendering of constraints in describe()

Author: nicoladicicco

src/constraints/channel.jl

@@ -1,4 +1,4 @@
-const description = """
+const description_channel = """
 The channel constraint establishes a bijective correspondence between two sets of variables. This means that each value in the first set of variables corresponds to a unique value in the second set, and vice versa.
 """
 
@@ -11,7 +11,7 @@ Return `true` if the channel constraint is satisfied, `false` otherwise. The cha
 - `list::Union{AbstractVector, Tuple}`: list of values to check.
 
 ## Variants
-- `:channel`: $description
+- `:channel`: $description_channel
 ```julia
 concept(:channel, x; dim=1, id=nothing)
 concept(:channel)(x; dim=1, id=nothing)

src/usual_constraints.jl

@@ -9,7 +9,7 @@ Adding a new constraint is as simple as defining a new function with the same na
 @usual concept_all_different(x; vals=nothing) = xcsp_all_different(list=x, except=vals)
 ```
 """
-const USUAL_CONSTRAINTS = Dict{Symbol, Constraint}(:always_true => Constraint())
+const USUAL_CONSTRAINTS = Dict{Symbol, Constraint}()
 
 """
     describe(constraints::Dict{Symbol,Constraint}=USUAL_CONSTRAINTS; width=150)
@@ -41,6 +41,8 @@ function describe(constraints::Dict{Symbol, Constraint} = USUAL_CONSTRAINTS; wid
     )
 end
 
+describe(s::Symbol) = USUAL_CONSTRAINTS[s].description
+
 """
     extract_parameters(s::Symbol, constraints_dict=USUAL_CONSTRAINTS; parameters=ConstraintCommons.USUAL_CONSTRAINT_PARAMETERS)