Added concat example

concat.modal

@@ -0,0 +1,8 @@
+define (: ?x ?y ;) (define ?x ?y)
+: (?x dup) (?x ?x) ;
+: (?x ?y swap) (?y ?x) ;
+: (?x drop) () ;
+: (?x ?y p*) (?x * ?y) ;
+: square (dup p*) ;
+
+10 square

prelude2.modal

@@ -0,0 +1,141 @@
+define [-- ?x] {}
+
+define [form ?x ?y] {
+    define ?x ?y
+}
+
+define [rule ?x ?y] {
+    define ?x ?y
+}
+
+define [?x -> ?y] {
+    define ?x ?y
+}
+
+[(?x) = (?x)] -> {
+    true
+}
+
+[(?x) = (?y)] -> {
+    false
+}
+
+[false or false] -> {
+    false
+}
+
+[true or false] -> {
+    true
+}
+
+[false or true] -> {
+    true
+}
+
+[true  or true] -> {
+    true
+}
+
+[quote ?x] -> {
+    quote ?x
+}
+
+[unquote (quote ?x)] -> {
+    ?x
+}
+
+form [
+    if ?condition
+            ?true
+        else
+            ?false
+]
+{
+    if/else ?condition {
+        quote ?true
+    } {
+        quote ?false
+    }
+}
+
+form [
+    if ?condition
+        ?branch
+]
+{
+    if/q ?condition {
+        quote ?branch
+    }
+}
+
+rule [
+    if/q (true)
+        ?branch
+]
+{
+    unquote ?branch
+}
+
+rule [
+    if/q (false)
+        ?branch
+]
+{}
+
+rule [
+    if/else (true)
+        ?true
+        ?false
+]
+{
+    unquote ?true
+}
+
+rule [
+    if/else (false)
+        ?true
+        ?false
+]
+{
+    unquote ?false
+}
+
+
+[factorial (?x)] -> {
+    if ((?x) = (1)) {
+        1
+    }
+    else {
+        ?x * factorial (?x - 1)
+    }
+}
+
+[fibonacci (?number)] -> {
+    if((?number) = (0) or (?number) = (1)) {
+        ?number
+    }
+    else {
+        fibonacci (?number - 1) + fibonacci (?number - 2)
+    }
+}
+
+[range (?low) (?high)] -> {
+    if((?low) = (?high + 1)) {
+        nil
+    }
+    else {
+        ?low : range (?low + 1) (?high)
+    }
+}
+
+[fold (?operation) (?initial) (nil)] -> {
+    ?initial
+}
+
+[fold (?operation) (?initial) (?head : ?tail)] -> {
+    ?operation ?head (fold (?operation) (?initial) ?tail)
+}
+
+[sum ?list] -> {
+    fold (add) (0) ?list
+}