Here is a contribution that enables %< and %> to escape code execution. 
This would be useful for anything that uses #lambda, as well as a number 
of other things. When %< is enabled the only %-substitution that will be 
parsed is %> the closing quote.

th [map(#lambda/\[inc\(\%0\)\],lnum(10))]

becomes

th [map(#lambda/%<[inc(%0)]%>,lnum(10))]

You can tell it's a lot more readable, especially for large blocks of 
code. I know this really isn't a substition, but I think it fits great 
and would be a great asset.

- grapenut

Here's the code:

 From parse.h:
#define PE_NOPERCENTSUBS        0x00000800

 From parse.c:
/* Global variable defined just before process_expression() to hold the 
eflags while quoting */
static int saveflags;

/* In process_expression() at the beginning of the %-substition block */
    case '%':                   /* Percent substitutions */
      /* If PE_NOPERCENTSUBS is set, only %> will be parsed. */
      if ((eflags & PE_NOPERCENTSUBS)) {
        char savec;
       
        (*str)++; 
        savec = **str;
        if (!savec)
          goto exit_sequence;
       
        if (savec == '>') {
          if (saveflags)
            eflags = saveflags;
          else
            eflags = PE_DEFAULT;
          (*str)++;
        } else {
          safe_chr('%', buff, bp);
        }
      } else if (!(eflags & PE_EVALUATE) || (*bp - buff >= BUFFER_LEN - 
1)) {

/* Further down in the % substition switch() */
        switch (savec) {
        case '<':               /* %< - start literal quoting, except 
for % subs */
          saveflags = eflags;  
          eflags = (eflags & ~PE_FUNCTION_CHECK) | PE_LITERAL | 
PE_NOPERCENTSUBS;
          break;
        case '>':               /* %> - stop literal quoting, this is 
handled above */
          break;
        case '%':               /* %% - a real % */
          safe_chr('%', buff, bp);
          break;