This is METAFONT, WAITS Version 0.6 (preloaded base=plain 84.12.20)  22 DEC 1984 21:37
**expr
(EXPR.MF[MF,SYS]
gimme an expr: hide(fill unitsquare) currentpicture
>> Edge structure at line 5:
row 0: 0+ 1- |

gimme an expr: hide(picture V; fill unitsquare scaled 2; V=currentpicture)V
>> Edge structure at line 5:
row 1: 0+ 2- |
row 0: 0+ 2- 0+ 1- |

gimme an expr: V rotated 45
! That transformation is too hard.
<to be read again> 
                   ;
<forever> ...string;showall.scantokens.s0;
                                           ENDFOR
l.5 showall scantokens s0; endfor
                                 
I can apply complicated transformations to paths,
but I can only do fairly simple things to pictures. Sorry.
Proceed, and I'll omit the transformation.

>> Edge structure at line 5:
row 1: 0+ 2- |
row 0: 0+ 2- 0+ 1- |

gimme an expr: V scaled 2
! That transformation is too hard.
<to be read again> 
                   ;
<forever> ...string;showall.scantokens.s0;
                                           ENDFOR
l.5 showall scantokens s0; endfor
                                 
I can apply complicated transformations to paths,
but I can only do fairly simple things to pictures. Sorry.
Proceed, and I'll omit the transformation.

>> Edge structure at line 5:
row 1: 0+ 2- |
row 0: 0+ 2- 0+ 1- |

gimme an expr: V scaled -1
>> Edge structure at line 5:
row -1: 0- -2+ 0- -1+ |
row -2: 0- -2+ |

gimme an expr: V rotated 180
>> Edge structure at line 5:
row -1: 0- -2+ 0- -1+ |
row -2: 0- -2+ |

gimme an expr: V shifted (1.5,3.14159)
>> Edge structure at line 5:
row 4: 2+ 4- |
row 3: 2+ 4- 2+ 3- |

gimme an expr: end
! An expression can't begin with `end'.
<inserted text> 
                0
<to be read again> 
                   end
<scantokens>  end
                 
<to be read again> 
                   ;
<forever> ...string;showall.scantokens.s0;
                                           ENDFOR
l.5 showall scantokens s0; endfor
                                 
I'm afraid I need some sort of value in order to continue,
so I've tentatively inserted `0'. You may want to
delete this zero and insert something else;
see Chapter xx of The METAFONTbook for an example.

>> 0