prelude: add an example for transform

prelude.tcl

@@ -4,11 +4,15 @@ namespace import tcl::mathfunc::max
 
 proc {#} args {}
 
+# short aliases for common things
+
 proc llen {lst} { return [llength $lst] }
 proc slen {str} { return [string length $str] }
 proc trim args { return [uplevel [concat string trim $args]] }
 proc replace {str a b} { return [string map [list $a $b] $str] }
 
+# sum and product
+
 proc ladd {list} {
     set t 0
     foreach x $list { incr t $x }
@@ -33,7 +37,13 @@ proc must_regexp args {
     }
 }
 
-# transpose an array of strings
+# transpose a list of strings
+#
+#     % transpose {abc 123}
+#     a1 b2 c3
+#     % transpose a1 b2 c3
+#     abc 123
+#
 proc transpose strings {
     set out {}
     set C [slen [lindex $strings 0]]