( 
df-addr -- ) ( F: 
r -- ) or
      ( 
r df-addr -- )
      
      Store the floating-point number 
r as a 64-bit IEEE
      double-precision number at 
df-addr. If the significand
      of the internal representation of 
r has more precision than
      the IEEE double-precision format, it will be rounded using the
      "round to nearest" rule. An ambiguous condition exists if the
      exponent of 
r is too large to be accommodated in IEEE
      double-precision format.