My bet would be:
alias +weld "weapon_welder; +attack"
alias -weld "-attack; lastinv"
bind <key> +weld
Not a bad script per se, makes the welder work sort of like the flashlight in doom 3. Press a key and it pops out and starts welding, release the key and it switches back to your last weapon. But definitely won't work with blockscripts on.