Convert a variable with elements {list lements + normal elems} to a string.

I have a variable:

set a { 1} { 9} 10 12 13

here all single digits are in this format: Brace+ space+digit+brace

I have to convert this to a: 1 9 10 12 13

That is, I have to print the variable a with no space that precedes the single digit and should remove braces.

Please help me.