The number x is converted to a string, prec is the
number of significant digits (def. 6). len is the
length of the output field, if len 0 the output
is left aligned.
SPECTRA chooses a f-format for numbers with
abs(x) > 0.1 and abs(x) 10000000.
e.g.:
SPECTRA> x = 1.234567
SPECTRA> say "Test "fcvt(x/100, 3, -10)
test 0.012