The function max_print_depth is used to define the maximum depth of
nesting that the pretty printer will allow. If the number of blocks
is greater than the the value set by max_print_depth then
the blocks are truncated and this is indicated by the holophrast &.
The function always returns the previous maximum depth setting.
FAILURE
Never fails.
EXAMPLE
If the maximum depth setting is the default (500) and we want to
change this to 20 the command will be: