PARI 2.6 syntax 2: [f(x)|x<-a,b(x)]

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

Dear PARI developers,

In PARI 2.6 I introduced the following syntax:
[f(x)|x<-V] and
[f(x)|x<-V,f(x)] 

for example [x^2|x<-[1..100],isprime(x)]
returns the list of the square of the prime number less than 100.

The symbol <- was chosen because it was similar to Haskell syntax, and
looked similar to both 'element of' ∈ and to a left arrow ←, so it 
accomodates both mathematicians and computer scientists faith.

Unfortunately the choice of the symbol <- is problematic because
this leads to an ambiguity between x (<-) a and x < (-a), so we can
only use it immediatly after a vertical bar |, which prevents generalizing
the syntax.

Karim proposes to use <~ instead which is not ambiguous since ~ is not
a valid prefix operator. Unfortunately it is ugly with some fonts where
~ is displayed at the top instead of the middle (looking  like in <˜). Also
it is a 'dead key' on some system (but we already use it for transposition
anyway). 

Pascal proposed to use the euro symbol €, but people using US keyboard might
find it hard to type.

Any other suggestion ?

Cheers,
Bill.