Add tasks for all the new languages

Task/Stack/Phix/stack-1.phix

@@ -0,0 +1,23 @@
+sequence stack = {}
+
+procedure push(object what)
+    stack = append(stack,what)
+end procedure
+
+function pop()
+    object what = stack[$]
+    stack = stack[1..$-1]
+    return what
+end function
+
+function empty()
+    return length(stack)=0
+end function
+
+?empty()                -- 1
+push(5)
+?empty()                -- 0
+push(6)
+?pop()                  -- 6
+?pop()                  -- 5
+?empty()                -- 1

Task/Stack/Phix/stack-2.phix

@@ -0,0 +1,26 @@
+function push(sequence stack, object what)
+    stack = append(stack,what)
+    return stack
+end function
+
+function pop(sequence stack)
+    object what = stack[$]
+    stack = stack[1..$-1]
+    return {stack,what}
+end function
+
+function empty(sequence stack)
+    return length(stack)=0
+end function
+
+sequence stack = {}
+?empty(stack)               -- 1
+stack = push(stack,5)
+?empty(stack)               -- 0
+stack = push(stack,6)
+integer top
+{stack,top} = pop(stack)
+?top                        -- 6
+{stack,top} = pop(stack)
+?top                        -- 5
+?empty(stack)               -- 1

Task/Stack/Phix/stack-3.phix

@@ -0,0 +1,43 @@
+sequence stacks = {}
+integer freelist = 0
+
+function new_stack()
+integer res = freelist
+    if res!=0 then
+        freelist = stacks[freelist]
+        stacks[res] = {}
+    else
+        stacks = append(stacks,{})
+        res = length(stacks)
+    end if
+    return res
+end function
+
+procedure free_stack(integer sid)
+    stacks[sid] = freelist
+    freelist = sid
+end procedure
+
+procedure push(integer sid, object what)
+    stacks[sid] = append(stacks[sid],what)
+end procedure
+
+function pop(integer sid)
+    object res = stacks[sid][$]
+    stacks[sid] = stacks[sid][1..$-1]
+    return res
+end function
+
+function empty(integer sid)
+    return length(stacks[sid])=0
+end function
+
+integer sid = new_stack()
+?empty(sid)                 -- 1
+push(sid,5)
+?empty(sid)                 -- 0
+push(sid,6)
+?pop(sid)                   -- 6
+?pop(sid)                   -- 5
+?empty(sid)                 -- 1
+free_stack(sid)