Thanks, Favonia. Meanwhile I solved this as in the following commit (file  
source/UF-Subsingletons-FunExt.lagda 
<https://github.com/martinescardo/TypeTopology/commit/7433e08de497216cbe131727c4a367eaed85847e#diff-562e7978f09d797d06b9bc40fc2e0c0e>), https://github.com/martinescardo/TypeTopology/commit/7433e08de497216cbe131727c4a367eaed85847e, 
which may be similar to what you are saying. However, the problem with such 
a solution is that it has to be specialized to each situation where we have 
inputs defined with some ex/implicit arguments which we want to apply to a 
function defined with some other ex/implicit arguments. In my view, a 
definition with implicit arguments should be considered to be the same as 
the definition with explicit arguments, as in real-life informal 
mathematics. Best, Martin  

On Tuesday, 26 June 2018 20:16:06 UTC+1, Favonia wrote:
>
> Hi Martin,
>
> I don't know your definition of is-prop, but how about this?
>
> open import Agda.Primitive
>
> _* : ∀ U → Set (lsuc U)
> U * = Set U
>
> data _≡_ {U} {X : U *} (x : X) : X → U * where
>   refl : x ≡ x
>
> is-prop : ∀ {U} → U * → U *
> is-prop X = (x y : X) → x ≡ y
>
> is-set : ∀ {U} → U * → U *
> is-set X = {x y : X} → is-prop (x ≡ y)
>
> is-set' : ∀ {U} → U * → U *
> is-set' X = (x y : X) → is-prop (x ≡ y)
>
> is-set'-is-set : ∀ {U} {X : U *} → is-set' X → is-set X
> is-set'-is-set s {x} {y} = s x y
>
> is-set-is-set' : ∀ {U} {X : U *} → is-set X → is-set' X
> is-set-is-set' s x y = s {x} {y}
>
> funext : ∀ U0 U1 → lsuc (U0 ⊔ U1) *
> funext U0 U1 = {X : U0 *} {Y : X → U1 *} (f g : (x : X) → Y x) → (∀ x → f 
> x ≡ g x) → f ≡ g
>
> postulate
>   is-prop-is-set' : ∀ {U} {X : U *} → funext U U → is-prop (is-set' X)
>
> ap : ∀ {U0 U1} {X : U0 *} {Y : U1 *} (f : X → Y) {x y : X} → x ≡ y → f x ≡ 
> f y
> ap f refl = refl
>
> is-prop-is-set : ∀ {U} {X : U *} → funext U U → is-prop (is-set X)
> is-prop-is-set fe isset0 isset1 =
>   ap is-set'-is-set (is-prop-is-set' fe (is-set-is-set' isset0) 
> (is-set-is-set' isset1))
>
> Best,
> Favonia
>
> On Wed, Jun 20, 2018 at 3:46 PM Martín Hötzel Escardó <
> escardo...@gmail.com <javascript:>> wrote:
>
>> Bad copy and paste. Let me fix this.
>>
>>
>> is-set : ∀ {U} → U ̇ → U ̇
>> is-set X = {x y : X} → is-prop(x ≡ y)
>>
>> is-set' : ∀ {U} → U ̇ → U ̇
>> is-set' X = (x y : X) → is-prop(x ≡ y)
>>
>> Martin
>>
>>>
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