what do you think of the gbook mixin?

*** ERROR: ERROR IN MIXIN -- UNKNOWN usr COMMAND: ::andy::FieldKeysMTS UNKNOWN usr COMMAND: ::andy::FieldKeysMTS    while executing "error "UNKNOWN usr COMMAND: $cmd""    (procedure "::unknown" line 32)    invoked from within "^FieldKeysMTS gbook 447"    ("uplevel" body line 1)    invoked from within "uplevel 1 [list ^$cmd] $args "    (procedure "::unknown" line 30)    invoked from within "^FieldKeysMTS gbook $::Page "    (procedure "::demo_gbook::Slant/demo_gbook" line 5)    invoked from within "^Slant/demo_gbook {TELL US NOW}"    ("uplevel" body line 1)    invoked from within "uplevel 1 [list ^$cmd] $args "    (procedure "::unknown" line 26)    invoked from within "^Slant/demo_gbook {TELL US NOW}"    ("eval" body line 1)    invoked from within "eval ^Slant/demo_gbook {{TELL US NOW}}" *** LINE: /demo_gbook TELL US NOW ***