Enable input over multiple lines in Dyalog APL session

To activate, execute the following (were <path> is the location of MultiLine.dyalog):

#.⎕TRAP∪⍨←⊂2 'E' '⎕THIS ⎕SE.MultiLine ⎕IO⊃1↓⎕DM'⊣2⎕SE.⎕FIX'file://<path>/MultiLine.dyalog'

You can now use multi-line input in the session, for example:

      f←{
          ⍺ ⍵
      }
      2 f 3
5
      :if 1
          'one'
      :else
          'zero'
      :endif
one
      2{
          ⍺ ⍵
      }3
5
      :namespace foo
          goo← 
      :endnamespace
      2 foo.goo 3
5

To cancel input, enter a lone →.

Does not handle :Until followed by :AndIf/:OrIf.

Does not handle diamonds (⋄); use newlines instead.

Errors are re-signalled with error number 500 ⎕EN and the code line will have the trap statement instead of the offending line.